Remove POST method from password reset submit_token endpoint (#6056)

Removes the POST method from `/password_reset/<medium>/submit_token/` as it's only used by phone number verification which Synapse does not support yet.

changelog.d/6056.bugfix

@@ -0,0 +1 @@
+Remove POST method from password reset submit_token endpoint until we implement submit_url functionality.

synapse/rest/client/v2_alpha/account.py

@@ -272,23 +272,6 @@ def on_GET(self, request, medium):
         request.write(html.encode("utf-8"))
         finish_request(request)
 
-    @defer.inlineCallbacks
-    def on_POST(self, request, medium):
-        if medium != "email":
-            raise SynapseError(
-                400, "This medium is currently not supported for password resets"
-            )
-
-        body = parse_json_object_from_request(request)
-        assert_params_in_dict(body, ["sid", "client_secret", "token"])
-
-        valid, _ = yield self.store.validate_threepid_session(
-            body["sid"], body["client_secret"], body["token"], self.clock.time_msec()
-        )
-        response_code = 200 if valid else 400
-
-        return response_code, {"success": valid}
-
 
 class PasswordRestServlet(RestServlet):
     PATTERNS = client_patterns("/account/password$")