Clean up comments in defaults.yaml and macro-scm.yaml

* Remove commented out "# defaults: global".

* Fix "hardcodes" typo.

* Add FIXME pointing out that git-mwcore and git-mwcore-nosubmodules
  are the same (both have submodules disabled).

Change-Id: I3b5c00d22ad0b92634240908a2787fec40b9d3a9