Mk/Uses: remove the default deprecation date.

Obviously the world did not manage to get rid of Python 2.7
by 2020-12-31, so remove that target and just advise users
to move on to Python 3.

This is the commit 1/9 of review D33922

Differential Revision: https://reviews.freebsd.org/D33922

(cherry picked from commit fcb5bf842e0424b9fb70f3e489d9cc39f3c857be)