Fix federation lookups in onbind callbacks

It was doing an extra pickServer before attempting to hit the
matrix URL, so effectively doing the srv lookup twice, which makes
it not work. This removes all the unnecessary code and makes it work.

Fixes element-hq/element-web#10483

sydent/threepid/bind.py

@@ -100,11 +100,16 @@ def removeBinding(self, threepid, mxid):
     @defer.inlineCallbacks
     def _notify(self, assoc, attempt):
         mxid = assoc["mxid"]
-        domain = mxid.split(":")[-1]
-        server = yield self._pickServer(domain)
+        mxid_parts = mxid.split(":", 1)
+        if len(mxid_parts) != 2:
+            logger.error(
+                "Can't notify on bind for unparseable mxid %s. Not retrying.",
+                assoc["mxid"],
+            )
+            return
 
-        post_url = "https://%s/_matrix/federation/v1/3pid/onbind" % (
-            server,
+        post_url = "matrix://%s/_matrix/federation/v1/3pid/onbind" % (
+            mxid_parts[1],
         )
 
         logger.info("Making bind callback to: %s", post_url)
@@ -146,56 +151,3 @@ def _notifyErrback(self, assoc, attempt, error):
     # The below is lovingly ripped off of synapse/http/endpoint.py
 
     _Server = collections.namedtuple("_Server", "priority weight host port")
-
-    @defer.inlineCallbacks
-    def _pickServer(self, host):
-        servers = yield self._fetchServers(host)
-        if not servers:
-            defer.returnValue("%s:8448" % (host,))
-
-        min_priority = servers[0].priority
-        weight_indexes = list(
-            (index, server.weight + 1)
-            for index, server in enumerate(servers)
-            if server.priority == min_priority
-        )
-
-        total_weight = sum(weight for index, weight in weight_indexes)
-        target_weight = random.randint(0, total_weight)
-
-        for index, weight in weight_indexes:
-            target_weight -= weight
-            if target_weight <= 0:
-                server = servers[index]
-                defer.returnValue("%s:%d" % (server.host, server.port,))
-                return
-
-    @defer.inlineCallbacks
-    def _fetchServers(self, host):
-        try:
-            service = "_matrix._tcp.%s" % host
-            answers, auth, add = yield client.lookupService(service)
-        except DNSNameError:
-            answers = []
-
-        if (len(answers) == 1
-                and answers[0].type == dns.SRV
-                and answers[0].payload
-                and answers[0].payload.target == dns.Name(".")):
-            raise DNSNameError("Service %s unavailable", service)
-
-        servers = []
-
-        for answer in answers:
-            if answer.type != dns.SRV or not answer.payload:
-                continue
-            payload = answer.payload
-            servers.append(ThreepidBinder._Server(
-                host=str(payload.target),
-                port=int(payload.port),
-                priority=int(payload.priority),
-                weight=int(payload.weight)
-            ))
-
-        servers.sort()
-        defer.returnValue(servers)