linux-yocto/5.13: drop recipes

5.13 is EOL upstream, and we've moved to 5.14 for the release, so
we drop the versioned recipes. The kernel tree and branches will
stay available for use, but other layers must create recipes if
they wish to continue with 5.13.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>