mk: LZMA_SUPPORT is unused

Retire LZMA_SUPPORT. It's unused since r332995.

Reviewed by: delphij
PR: 244302
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D31088

(cherry picked from commit d8514fa6f1b0b9824b169c5ab66f37713b303c57)