sched_ext: Drop kfuncs marked for removal in 6.15

sched_ext: Drop kfuncs marked for removal in 6.15

sched_ext performed a kfunc renaming pass in 6.13 and kept the old names
around for compatibility with old binaries. These were scheduled for
cleanup in 6.15 but were missed. Submitting for cleanup in for-next.

Removed the kfuncs, their flags, and any references I could find to them
in doc comments. Left the entries in include/scx/compat.bpf.h as they're
still useful to make new binaries compatible with old kernels.

Tested by applying to my kernel. It builds and a modern version of
scx_lavd loads fine.

Signed-off-by: Jake Hillion <jake@hillion.co.uk>
Signed-off-by: Tejun Heo <tj@kernel.org>