Eliminate thread pool mutex

[nicoburns]

This was quite straightforward (just remove the Mutex and the calls to the lock methods). And StyleThreadPool already internally contains an RwLock. Perhaps this was previously required but isn't anymore?

Reduces the diff with upstream which doesn't have this Mutex.

Servo PR:

• Move stylo thread pool mutex to servo layout thread crates servo#34480

[nicoburns]

It seems this doesn't work. Perhaps we could:

• Pop a comment into the source code linking to this analysis
• Move the Mutex into Servo layout_thread crates so that we don't need to maintain the diff here.

?

[jdm]

If it's possible to move it into Servo's layout thread code, that seems like a worthwhile change.

[nicoburns]

If it's possible to move it into Servo's layout thread code, that seems like a worthwhile change.

@jdm Implemented over in servo/servo#34480. I'll run the test suite again to check it works.