push: remove "push.default is unset" warning message

push: remove "push.default is unset" warning message

The warning was important before the 2.0 transition, and remained
important for a while after, so that new users get push.default
explicitly in their configuration and do not experience inconsistent
behavior if they ever used an older version of Git.

The warning has been there since version 1.8.0 (Oct 2012), hence we can
expect the vast majority of current Git users to have been exposed to
it, and most of them have already set push.default explicitly. The
switch from 'matching' to 'simple' was planned for 2.0 (May 2014), but
actually happened only for 2.3 (Feb 2015).

Today, the warning is mostly seen by beginners, who have not set their
push.default configuration (yet). For many of them, the warning is
confusing because it talks about concepts that they have not learned and
asks them a choice that they are not able to make yet. See for example

  http://stackoverflow.com/questions/13148066/warning-push-default-is-unset-its-implicit-value-is-changing-in-git-2-0

(1260 votes for the question, 1824 for the answer as of writing)

Remove the warning completely to avoid disturbing beginners. People who
still occasionally use an older version of Git will be exposed to the
warning through this old version.

Eventually, versions of Git without the warning will be deployed enough
and tutorials will not need to advise setting push.default anymore.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>