Bug 1524327 remove MOZ_ALLOW_LEGACY_EXTENSIONS and extensions.legacy.enabled r=zombie,aswan

Legacy extensions are no longer loaded, so we can drop the build config for it.  We
still need flags for handling experimental APIs since what we require differs between builds
and distributions.

Differential Revision: https://phabricator.services.mozilla.com/D57413

--HG--
extra : moz-landing-system : lando