Remove SMALL conditionals from gzip

gzip has SMALL conditionals which enable building a reduced size version
of the binary. These exist as part of the introduction of BSD licensed
gzip in 2004 in NetBSD and appear to have been required to reach a size
for inclusion in their install media. For more information see commits
to gzip in the NetBSD tree on the 28th of March 2004.

SMALL doesn't appear to be hooked up to our build system and
complicates gzip quite a bit.

Reviewed by: kevans, imp
Sponsored by: Klara Inc.
Differential Revision: https://reviews.freebsd.org/D34047

(cherry picked from commit 4669f23ef76bd5ff7d122546ca5ed3e8575f1c73)