Remove minigzip from the base system.

The minigzip(1) was initially introduced as a minimal replacement
of GNU gzip(1). We have replaced the GNU gzip(1) with the zlib
based, GPL-free implementation, and minigzip(1) is not being used
anywhere in the base system, so just remove it.

Relnotes: yes
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D35979