advice: Remove unused advice_push_non_ff_default

Commit 27f76a2b ('push: switch default from "matching" to "simple"',
04-01-2013) removed the last use of the 'advice_push_non_ff_default'
variable, along with the advice message which it used to suppress.

Remove the 'advice_push_non_ff_default' variable definition, along
with the now redundant advice.pushnonffdefault config variable
(including it's documentation).

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>