Disable building svnlite(1) by default.

Now that all repositories have switched to git, initiate the de-orbit
burn for svnlite(1).

Reviewed by: emaste
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D30105

(cherry picked from commit a2bc17474b962ba9e29c4526356203fe48a549eb)