Remove redundant types from comments. (#14412)

Remove type hints from comments which have been added
as Python type hints. This helps avoid drift between comments
and reality, as well as removing redundant information.

Also adds some missing type hints which were simple to fill in.

synapse/replication/http/_base.py

@@ -153,7 +153,7 @@ class ReplicationEndpoint(metaclass=abc.ABCMeta):
         argument list.
 
         Returns:
-            dict: If POST/PUT request then dictionary must be JSON serialisable,
+            If POST/PUT request then dictionary must be JSON serialisable,
             otherwise must be appropriate for adding as query args.
         """
         return {}