contrib: Remove now-unused cortex-strings
Replaced by arm-optimized-routines in commit 2e8ff4d1d5b9 ("Switch to
Arm Optimized Routines for mem* & str*").
Reviewed by: andrew
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48500
(cherry picked from commit 4a7d6666c5ddd135b2023b4b3b4962770ff2e73b)