Remove the ability to send state events that are MSC4354 sticky (#20256)

Part of: MSC4354 whose Experimental Feature tracking issue is
https://github.com/element-hq/synapse/issues/19409

Remove the ability to create sticky (MSC4354) state events as a client
This was removed from the MSC in

https://github.com/matrix-org/matrix-spec-proposals/commit/d7d1546d3aa2323ec5d97a50c9f61ab92cd9e14b

---------

Signed-off-by: Olivier 'reivilibre <oliverw@matrix.org>

changelog.d/20256.removal

@@ -0,0 +1 @@
+Remove the ability to send state events that are sticky, as this feature was removed from [MSC4354: Sticky Events](https://github.com/matrix-org/matrix-spec-proposals/pull/4354).

synapse/rest/client/room.py

@@ -217,7 +217,6 @@ class RoomStateEventRestServlet(RestServlet):
         self.clock = hs.get_clock()
         self._event_serializer = hs.get_event_client_serializer()
         self._spam_checker_module_callbacks = hs.get_module_api_callbacks().spam_checker
-        self._msc4354_enabled = hs.config.experimental.msc4354_enabled
 
     def register(self, http_server: HttpServer) -> None:
         # /rooms/$roomid/state/$eventtype
@@ -339,10 +338,6 @@ class RoomStateEventRestServlet(RestServlet):
         if requester.app_service_id:
             origin_server_ts = parse_integer(request, "ts")
 
-        sticky_duration_ms: int | None = None
-        if self._msc4354_enabled:
-            sticky_duration_ms = parse_integer(request, StickyEvent.QUERY_PARAM_NAME)
-
         delay = _parse_request_for_delayed_event_delay(request)
         if delay is not None:
             delay_id = await self.delayed_events_handler.add(
@@ -353,7 +348,7 @@ class RoomStateEventRestServlet(RestServlet):
                 origin_server_ts=origin_server_ts,
                 content=content,
                 delay=delay,
-                sticky_duration_ms=sticky_duration_ms,
+                sticky_duration_ms=None,
             )
 
             set_tag("delay_id", delay_id)
@@ -381,10 +376,6 @@ class RoomStateEventRestServlet(RestServlet):
                     "room_id": room_id,
                     "sender": requester.user.to_string(),
                 }
-                if sticky_duration_ms is not None:
-                    event_dict[StickyEvent.EVENT_FIELD_NAME] = StickyEventField(
-                        duration_ms=sticky_duration_ms
-                    )
 
                 if state_key is not None:
                     event_dict["state_key"] = state_key