Add /account/3pid/delete endpoint

Also fix a typo in a comment

synapse/storage/registration.py

@@ -413,6 +413,17 @@ class RegistrationStore(background_updates.BackgroundUpdateStore):
             desc="user_delete_threepids",
         )
 
+    def user_delete_threepid(self, user_id, medium, address):
+        return self._simple_delete(
+            "user_threepids",
+            keyvalues={
+                "user_id": user_id,
+                "medium": medium,
+                "address": address,
+            },
+            desc="user_delete_threepids",
+        )
+
     @defer.inlineCallbacks
     def count_all_users(self):
         """Counts all users registered on the homeserver."""