Retire MK_PROFILE infrastructure

[emaste]

Summary

We announced that the option would be removed before FreeBSD 14; it was
first disabled by default in fe52b7f60ef4.

Relnotes:       Yes
Sponsored by:   The FreeBSD Foundation

[emaste]

TODO: determine if we can/should reuse any of this for code coverage (idea from @phk)

[emaste]

contrib/bmake/mk/lib.mk

258–260

@sjg, this would have no effect after my change but would you want to leave it to reduce diffs against upstream?

[emaste]

I changed the WITH_PROFILE description in f768ecf247b7acd98fd0ee395522e559769e3e51 as it may not be gone in 14.