Remove polylith/API mode (#2967)

This removes most of the code used for polylith/API mode.

This removes the `/api` internal endpoints entirely. 

Binary size change roughly 5%: 
```
51437560 Feb 13 10:15 dendrite-monolith-server # old
48759008 Feb 13 10:15 dendrite-monolith-server # new
```

cmd/dendrite/main.go

@@ -0,0 +1,109 @@
+// Copyright 2017 Vector Creations Ltd
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"flag"
+
+	"github.com/sirupsen/logrus"
+
+	"github.com/matrix-org/dendrite/appservice"
+	"github.com/matrix-org/dendrite/federationapi"
+	"github.com/matrix-org/dendrite/keyserver"
+	"github.com/matrix-org/dendrite/roomserver"
+	"github.com/matrix-org/dendrite/setup"
+	basepkg "github.com/matrix-org/dendrite/setup/base"
+	"github.com/matrix-org/dendrite/setup/config"
+	"github.com/matrix-org/dendrite/setup/mscs"
+	"github.com/matrix-org/dendrite/userapi"
+)
+
+var (
+	httpBindAddr  = flag.String("http-bind-address", ":8008", "The HTTP listening port for the server")
+	httpsBindAddr = flag.String("https-bind-address", ":8448", "The HTTPS listening port for the server")
+	certFile      = flag.String("tls-cert", "", "The PEM formatted X509 certificate to use for TLS")
+	keyFile       = flag.String("tls-key", "", "The PEM private key to use for TLS")
+)
+
+func main() {
+	cfg := setup.ParseFlags(true)
+	httpAddr := config.HTTPAddress("http://" + *httpBindAddr)
+	httpsAddr := config.HTTPAddress("https://" + *httpsBindAddr)
+	options := []basepkg.BaseDendriteOptions{}
+
+	base := basepkg.NewBaseDendrite(cfg, options...)
+	defer base.Close() // nolint: errcheck
+
+	federation := base.CreateFederationClient()
+
+	rsAPI := roomserver.NewInternalAPI(base)
+
+	fsAPI := federationapi.NewInternalAPI(
+		base, federation, rsAPI, base.Caches, nil, false,
+	)
+
+	keyRing := fsAPI.KeyRing()
+
+	keyAPI := keyserver.NewInternalAPI(base, &base.Cfg.KeyServer, fsAPI, rsAPI)
+
+	pgClient := base.PushGatewayHTTPClient()
+	userAPI := userapi.NewInternalAPI(base, &cfg.UserAPI, cfg.Derived.ApplicationServices, keyAPI, rsAPI, pgClient)
+
+	asAPI := appservice.NewInternalAPI(base, userAPI, rsAPI)
+
+	// The underlying roomserver implementation needs to be able to call the fedsender.
+	// This is different to rsAPI which can be the http client which doesn't need this
+	// dependency. Other components also need updating after their dependencies are up.
+	rsAPI.SetFederationAPI(fsAPI, keyRing)
+	rsAPI.SetAppserviceAPI(asAPI)
+	rsAPI.SetUserAPI(userAPI)
+	keyAPI.SetUserAPI(userAPI)
+
+	monolith := setup.Monolith{
+		Config:    base.Cfg,
+		Client:    base.CreateClient(),
+		FedClient: federation,
+		KeyRing:   keyRing,
+
+		AppserviceAPI: asAPI,
+		// always use the concrete impl here even in -http mode because adding public routes
+		// must be done on the concrete impl not an HTTP client else fedapi will call itself
+		FederationAPI: fsAPI,
+		RoomserverAPI: rsAPI,
+		UserAPI:       userAPI,
+		KeyAPI:        keyAPI,
+	}
+	monolith.AddAllPublicRoutes(base)
+
+	if len(base.Cfg.MSCs.MSCs) > 0 {
+		if err := mscs.Enable(base, &monolith); err != nil {
+			logrus.WithError(err).Fatalf("Failed to enable MSCs")
+		}
+	}
+
+	// Expose the matrix APIs directly rather than putting them under a /api path.
+	go func() {
+		base.SetupAndServeHTTP(httpAddr, nil, nil)
+	}()
+	// Handle HTTPS if certificate and key are provided
+	if *certFile != "" && *keyFile != "" {
+		go func() {
+			base.SetupAndServeHTTP(httpsAddr, certFile, keyFile)
+		}()
+	}
+
+	// We want to block forever to let the HTTP and HTTPS handler serve the APIs
+	base.WaitForShutdown()
+}