libsoft: Remove

libsoft was a transition from the FreeBSD/armv6 10 (and earlier) with
'softfp' ABI (that is, hardware float, but passed as integer registers)
to the 'hardfp' ABI that was in FreeBSD/armv[67] in FreeBSD 11 and
newer. It's been off by default since it was created.

This was mostly used by people that wanted to do a source upgrade of
their system from FreeBSD 10 to 11 or from 11-current before the cutover
to 11-current after. This should not be confused with the full software
floating point implementation (that doesn't use the hardware floating
point instructions at all) that is used out of tree by at least one
company selling armv7 gear that has no FPU.

There's no longer a need for the transition, so retire it like should
likely have happened sometime before FreeBSD 12 was released 3 years
ago.

Sponsored by: Netflix
Suggestions by: jrtc27, jhb
Reviewed by: manu, mmel
Differential Revision: https://reviews.freebsd.org/D33761