Use p.wait() instead of p.communicate()

The latter calls the former anyway, and we don't need to fetch the output nor
send any stdin.

fdroidserver/common.py

@@ -1681,7 +1681,7 @@ def FDroidPopen(commands, cwd=None, shell=False, output=True):
 
         time.sleep(0.1)
 
-    p.communicate()
+    p.wait()
     result.returncode = p.returncode
     return result