Makefile.inc1: Remove svn support

[imp]

Summary

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

[brd]

Thanks!