push: do not give big warning when no preference is configured

push: do not give big warning when no preference is configured

If the message said "we will be changing the default in the future, so
this is to warn people who want to keep the current default what to do",
it would have made some sense, but as it stands, the message is merely an
unsolicited advertisement for a new feature, which it is not helpful at
all.  Squelch it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>