armv6: Remove support for building armv6

With it planned that armv7 will be the only 32-bit kernel when 15.0 is
released remove support for armv6.

Remove the top level build infrastructure. It was already removed from
universe, this just stops it from being built directly.

Reviewed by: mmel, emaste
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D45634