fml::scoped_nsobject and fml::scoped_nsprotocol classes are no longer
necessary now that the entire codebase has been migrated to ARC.
This also eliminates the fml_arc_unittests target which tests that code.
All references to that target in documentation, run_tests.py have been
removed. This target was used in various `et` build/query unit tests and
has been replaced with the fml_unittests target in those cases.
Issue: https://github.com/flutter/flutter/issues/137801