fix: remove accidental gitlinks for yoga source directories

Remove gitlink entries for .yoga-source and .yoga-tests that were
accidentally added to the git index. These directories contain cloned
upstream Yoga repositories with their own .git directories and should
be gitignored, not tracked as submodules.

This fixes the error:
  fatal: no submodule mapping found in .gitmodules for path
  'packages/yoga-layout/.yoga-source'

The directories remain on disk and are properly gitignored via the
patterns in .gitignore (packages/*/.yoga-source and
packages/*/.yoga-tests).

Author: jdalton