Kill off redundant SynapseRequestFactory (#6619)

We already get the Site via the Channel, so there's no need for a dedicated
RequestFactory: we can just use the right constructor.

tests/server.py

@@ -20,6 +20,7 @@ from twisted.python.failure import Failure
 from twisted.test.proto_helpers import AccumulatingProtocol, MemoryReactorClock
 from twisted.web.http import unquote
 from twisted.web.http_headers import Headers
+from twisted.web.server import Site
 
 from synapse.http.site import SynapseRequest
 from synapse.util import Clock
@@ -42,6 +43,7 @@ class FakeChannel(object):
     wire).
     """
 
+    site = attr.ib(type=Site)
     _reactor = attr.ib()
     result = attr.ib(default=attr.Factory(dict))
     _producer = None
@@ -176,9 +178,9 @@ def make_request(
         content = content.encode("utf8")
 
     site = FakeSite()
-    channel = FakeChannel(reactor)
+    channel = FakeChannel(site, reactor)
 
-    req = request(site, channel)
+    req = request(channel)
     req.process = lambda: b""
     req.content = BytesIO(content)
     req.postpath = list(map(unquote, path[1:].split(b"/")))