remove "force_build_tools" config option, closes #738

The `force_build_tools` config option was added a long time ago to
brute force the _build-tools_ version by trying to replace the value
in `build.gradle` files. This is never something that should be used
in production, since the app's build metadata should specify this kind
of thing. And now that we're moving towards _androguard_ for
everything except fdroid build and fdroid publish, _build-tools_ will
no longer even be used in the other commands.

fdroidserver/common.py

@@ -115,7 +115,6 @@ default_config = {
     },
     'cachedir': os.path.join(os.getenv('HOME'), '.cache', 'fdroidserver'),
     'build_tools': MINIMUM_AAPT_BUILD_TOOLS_VERSION,
-    'force_build_tools': False,
     'java_paths': None,
     'scan_binary': False,
     'ant': "ant",