linux-yocto: remove 5.8 recipes

5.8 is EOL upstream, and we now have 5.10 available as the
latest -stable kernel.

We'll keep 5.4, 5.10 and -dev (5.11) as the active kernels
for the upcoming release.

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