clone: do not export and unexport GIT_CONFIG

clone: do not export and unexport GIT_CONFIG

Earlier, dc87183 (use GIT_CONFIG only in "git config", not other
programs, 2008-06-30) made sure that the environment variable is
never used outside "git config", but "git clone", after creating a
directory for the new repository and until the init_db() function
populates its .git/ directory, exported the variable for no good
reason.  No hook will run from init_db() and more importantly no
hook can run until init_db() finishes creation of the new
repository, so it cannot be used by any invocation of "git config"
by definition.

Stop doing the useless export/unexport.

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