recipes: Remove PR = r0 from all recipes

* Remove all PR = "r0" from all .bb files in meta-oe repo. This was done
  with the command sed -e '/^PR.*=.*r0\"/d' meta*/recipes*/*/*.bb -i

* We've switching to the PR server, PR bumps are no longer needed and
  this saves people either accidentally bumping them or forgetting to
  remove the lines (r0 is the default anyway).

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>