Remove traces of optional_flags from crosstool

AFAIK all uses have been migrated to features, so they are not needed anymore.

RELNOTES: CppRules: optional_compiler_flag was removed from CROSSTOOL, use features instead.
PiperOrigin-RevId: 192277764
1 file changed
tree: 1a7306c5f59f1ef5b85f97360b829419fab35fe0
  1. third_party/