Remove Android x86 support

Mozilla dropped Android x86 builds.

While the build infrastructure is still in place (for now), 144 betas already don't have x86 apks.

If Mozilla isn't releasing for x86 anymore, we shouldn't either, as the product will be in an unsupported state and possibly there will be build errors they don't catch.

Bugzilla meta: https://bugzilla.mozilla.org/show_bug.cgi?id=1942134