Remove minigzip from the base system.

[delphij]

Summary

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

[emaste]

Can we add a note to minigzip.1 (and MFC it) first, mentioning that it is removed from 14.x and to use gzip instead?

[pauamma_gundo.com]

In D35979#817085, @emaste wrote:

Can we add a note to minigzip.1 (and MFC it) first, mentioning that it is removed from 14.x and to use gzip instead?

Seconded.

[delphij]

Rebase.

[pauamma_gundo.com]

LGTM once https://reviews.freebsd.org/D36000 is MFC'd.