Bug 1738401 - Remove -Wno-shadow warning suppressions. r=firefox-build-system-reviewers,glandium

-Wshadow warnings are not enabled globally, so these -Wno-shadow suppressions have no effect. I had intended to enable -Wshadow globally along with these suppressions in some directories (in bug 1272513), but that was blocked by other issues.

There are too many -Wshadow warnings (now over 2000) to realistically fix them all. We should remove all these unnecessary -Wno-shadow flags cluttering many moz.build files.

Differential Revision: https://phabricator.services.mozilla.com/D132289