t/Makefile: stop setting GIT_CONFIG

Once upon a time, the setting of GIT_CONFIG in the
environment could affect how tests ran. Commit 9c3796f (Fix
setting config variables with an alternative GIT_CONFIG,
2006-06-20) unconditionally set GIT_CONFIG in the Makefile
when running tests to give us a known starting point.

This is insufficient for running the tests outside of the
Makefile, however, and 8565d2d (Make tests independent of
global config files, 2007-02-15) later set GIT_CONFIG
directly in test-lib.sh. At that point the Makefile setting
was redundant, but we never removed it. Let's do so now.

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

t/Makefile

@@ -36,11 +36,11 @@ test: pre-clean $(TEST_LINT)
 	$(MAKE) aggregate-results-and-cleanup
 
 prove: pre-clean $(TEST_LINT)
-	@echo "*** prove ***"; GIT_CONFIG=.git/config $(PROVE) --exec '$(SHELL_PATH_SQ)' $(GIT_PROVE_OPTS) $(T) :: $(GIT_TEST_OPTS)
+	@echo "*** prove ***"; $(PROVE) --exec '$(SHELL_PATH_SQ)' $(GIT_PROVE_OPTS) $(T) :: $(GIT_TEST_OPTS)
 	$(MAKE) clean-except-prove-cache
 
 $(T):
-	@echo "*** $@ ***"; GIT_CONFIG=.git/config '$(SHELL_PATH_SQ)' $@ $(GIT_TEST_OPTS)
+	@echo "*** $@ ***"; '$(SHELL_PATH_SQ)' $@ $(GIT_TEST_OPTS)
 
 pre-clean:
 	$(RM) -r '$(TEST_RESULTS_DIRECTORY_SQ)'