Remove support for the `proposal-in-review` label

As it has now been removed from the https://github.com/matrix-org/matrix-spec-proposals repo. Current support for it causes the bot to [add/create the label](https://github.com/matrix-org/matrix-spec-proposals/pull/3026#event-7308599087) when it shouldn't be doing so.

Search the repository for the exact `proposal-in-review` label and trace the code path that handles or creates it. Remove the obsolete support and verify that the bot no longer adds or creates this label; no specific test file is mentioned in the issue.

Written by the indexing model from the issue text.