Bug 1834144 - Make stylo thread pool size configurable via pref rather than...

Bug 1834144 - Make stylo thread pool size configurable via pref rather than just env. r=smaug,perftest-reviewers,jgraham,afinder

Remove a variety of code which we don't use to change the pool size. We
can just use the pref as needed in the future.

Differential Revision: https://phabricator.services.mozilla.com/D178575