Auto merge of rust-lang#3370 - RalfJung:windows-sync, r=RalfJung

windows: remove support for slim rwlock

Since rust-lang/rust#121956 we don't need it any more, and we are generally short on Windows staff so reducing the amount of code we have to test and maintain sounds like a good idea.

The InitOnce stuff is still used by `thread_local_key::StaticKey` on 64bit windows-gnu.

Author: bors