config: eliminate config_exclusive_filename

This is a magic global variable that was intended as an
override to the usual git-config lookup process. Once upon a
time, you could specify GIT_CONFIG to any git program, and
it would look only at that file. This turned out to be
confusing and cause a lot of bugs for little gain. As a
result, dc87183 (Only use GIT_CONFIG in "git config", not
other programs, 2008-06-30) took this away for all callers
except git-config.

Since git-config no longer uses it either, the variable can
just go away. As the diff shows, nobody was setting to
anything except NULL, so we can just replace any sites where
it was read with NULL.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>