Makefile.inc1: Remove svn support

We don't need this, and we don't use this. It's left over from the svn
days. We stopped supporting svn as a project entirely when 12.x went
EOL. And VCS_REVSION isn't in any current ucl file or anywhere else in
the tree.

Sponsored by: Netflix
Reviewed by : kevans, brd
Differential Revision: https://reviews.freebsd.org/D52912