arm: Remove useless armv6/armv7 options

Cleanup another remnant of the armv4/armv5 support. Now that we always
define armv6 or armv7, these lines can be deleted (execpt hwpmc_armv7.c
which now needs just one line). Since we don't do anything different
between armv6 and armv7 from a config file selection point of view,
delete them from options.arm. We are extremely unlikely to grow anything
new here during the remaining lifetime of 32-bit arm in FreeBSD.

Sponsored by: Netflix
Reviewed by: kevans, andrew
Differential Revision: https://reviews.freebsd.org/D37069