linux-yocto/6.1: drop recipes

linux-yocto/6.1: drop recipes

6.8-lts + linux-yocto-dev will the active reference kernels in
the upcoming release. We've moved all of the preferred versions
to 6.8, so we can now drop the recipes for 6.1

6.1 will continue to be maintained and updated for released
and supported branches.

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