t/test-lib: drop redundant unset of GIT_CONFIG

This is already handled by the mass GIT_* unsetting added by
95a1d12 (tests: scrub environment of GIT_* variables,
2011-03-15).

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

t/test-lib.sh

@@ -649,7 +649,6 @@ else # normal case, use ../bin-wrappers only unless $with_dashes:
 	fi
 fi
 GIT_TEMPLATE_DIR="$GIT_BUILD_DIR"/templates/blt
-unset GIT_CONFIG
 GIT_CONFIG_NOSYSTEM=1
 GIT_ATTR_NOSYSTEM=1
 export PATH GIT_EXEC_PATH GIT_TEMPLATE_DIR GIT_CONFIG_NOSYSTEM GIT_ATTR_NOSYSTEM