Remove the unstable org.matrix.msc3202.device_id masquerading alias (#20192)

Part of: https://github.com/element-hq/synapse/issues/19415

Follow-up to #19033

TLDR: date to drop support for prefixed `org.matrix.msc3202.device_id`
parameter was 2026-01-01. This PR removes the prefixed param.

Synapse has accepted the stable `device_id` parameter since v1.141.0
(#19033).

Before:

```
GET /_matrix/client/v3/account/whoami?user_id=@alice:test&org.matrix.msc3202.device_id=DEVICEID  # appservice token
  200 {"user_id": "@alice:test", "is_guest": false, "device_id": "DEVICEID"}
```

After:

```
GET /_matrix/client/v3/account/whoami?user_id=@alice:test&org.matrix.msc3202.device_id=DEVICEID  # appservice token
  200 {"user_id": "@alice:test", "is_guest": false}
```

### Pull Request Checklist

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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`.".
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* [x] [Code
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correct (run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

changelog.d/20192.removal

@@ -0,0 +1 @@
+Remove support for the unstable `org.matrix.msc3202.device_id` query parameter for application service device masquerading.

synapse/api/auth/base.py

@@ -317,9 +317,6 @@ class BaseAuth:
         - The returned device ID, if present, has been checked to be a valid device ID
           for the returned user ID.
         """
-        # TODO: We can drop unstable support after 2026-01-01 (couple months after stable support)
-        UNSTABLE_DEVICE_ID_ARG_NAME = b"org.matrix.msc3202.device_id"
-
         app_service = self.store.get_app_service_by_token(access_token)
         if app_service is None:
             return None
@@ -340,9 +337,7 @@ class BaseAuth:
         else:
             effective_user_id = app_service.sender
 
-        effective_device_id_args = request.args.get(
-            b"device_id", request.args.get(UNSTABLE_DEVICE_ID_ARG_NAME)
-        )
+        effective_device_id_args = request.args.get(b"device_id")
         if effective_device_id_args:
             effective_device_id = effective_device_id_args[0].decode("utf8")
             # We only just set this so it can't be None!