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, 2 deletions

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

index ba1141bbe5..a1d089ebac 100644
--- a/synapse/rest/client/versions.py
+++ b/synapse/rest/client/versions.py
@@ -170,8 +170,6 @@ class VersionsRestServlet(RestServlet):
                     ),
                     # MSC4140: Delayed events
                     "org.matrix.msc4140": bool(self.config.server.max_event_delay_ms),
-                    # MSC4151: Report room API (Client-Server API)
-                    "org.matrix.msc4151": self.config.experimental.msc4151_enabled,
                     # Simplified sliding sync
                     "org.matrix.simplified_msc3575": msc3575_enabled,
                 },