Drop unstable MSC4151 implementation (#18052)

It's been rotated out of known clients, and should be safe for removal
now.

Fixes https://github.com/element-hq/synapse/issues/17373

### Pull Request Checklist

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [x] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct
(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

1 files changed, 0 insertions, 12 deletions

diff --git a/synapse/rest/client/reporting.py b/synapse/rest/client/reporting.py

index 949f077035..c5037be8b7 100644
--- a/synapse/rest/client/reporting.py
+++ b/synapse/rest/client/reporting.py
@@ -20,13 +20,11 @@
 #
 
 import logging
-import re
 from http import HTTPStatus
 from typing import TYPE_CHECKING, Tuple
 
 from synapse._pydantic_compat import StrictStr
 from synapse.api.errors import AuthError, Codes, NotFoundError, SynapseError
-from synapse.api.urls import CLIENT_API_PREFIX
 from synapse.http.server import HttpServer
 from synapse.http.servlet import (
     RestServlet,
@@ -127,16 +125,6 @@ class ReportRoomRestServlet(RestServlet):
         self.clock = hs.get_clock()
         self.store = hs.get_datastores().main
 
-        # TODO: Remove the unstable variant after 2-3 releases
-        # https://github.com/element-hq/synapse/issues/17373
-        if hs.config.experimental.msc4151_enabled:
-            self.PATTERNS.append(
-                re.compile(
-                    f"^{CLIENT_API_PREFIX}/unstable/org.matrix.msc4151"
-                    "/rooms/(?P<room_id>[^/]*)/report$"
-                )
-            )
-
     class PostBody(RequestBodyModel):
         reason: StrictStr