recipes: Remove PR = r0 from all recipes

Remove all PR = "r0" from all .bb files in oe-core. This was done
with the command sed -e '/^PR.*=.*r0\"/d' 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: Richard Purdie <richard.purdie@linuxfoundation.org>