config: drop support for GIT_CONFIG_NOGLOBAL

config: drop support for GIT_CONFIG_NOGLOBAL

Now that test-lib sets $HOME to protect against pollution from user
settings, GIT_CONFIG_NOGLOBAL is not needed for use by the test
suite any more.  And as luck would have it, a quick code search
reveals no other users in the wild.

This patch does not affect GIT_CONFIG_NOSYSTEM, which is still
needed.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>