le(4): deprecate and mark for removal before 15.0

We kept le(4) in the pre-12.0 purge because it was needed for Qemu/MIPS
(virtio networking didn't work) but the MIPS port has been removed.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D40683

(cherry picked from commit b95d2237af403049d48c23229aa8bace3594970e)