remove obsolete i386 MD memchr implementation

bde reports (in a reply to r351700 commit mail):

This uses scasb, which was last optimal on the 8086, or perhaps the
original i386.  On freefall, it is several times slower than the
naive translation of the naive C code.

Reported by: bde
Reviewed by: kib, markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21785