usr.bin: Remove intrinsic utilities

These utilities can only function correctly if implemented as shell
builtins and exist only because POSIX previously required them. As of
POSIX 2024, they have all been reclassified as intrinsic utilities and
are no longer required to exist in PATH. We can therefore retire them.
Cf. XBD 1.7, XRAT C.1.8, Austin Group bug 854.

Note that kill(1) is also considered an intrinsic utility (because
only the shell can interpret job IDs correctly), but we have a working
standalone implementation, which we will keep.

PR: 291686
Relnotes: yes
Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D54239