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/api/errors.py

@@ -713,7 +713,7 @@ class HttpResponseException(CodeMessageException):
         set to the reason code from the HTTP response.
 
         Returns:
-            SynapseError:
+            The error converted to a SynapseError.
         """
         # try to parse the body as json, to get better errcode/msg, but
         # default to M_UNKNOWN with the HTTP status as the error text