test-lib tests: remove dead GIT_TEST_FRAMEWORK_SELFTEST variable

Stop setting the GIT_TEST_FRAMEWORK_SELFTEST variable. This was originally needed
back in 4231d1ba99 (t0000: do not get self-test disrupted by
environment warnings, 2018-09-20).

It hasn't been needed since I deleted the relevant code in test-lib.sh
in c0eedbc009 (test-lib: remove check_var_migration, 2021-02-09), I
just didn't notice that it was set here.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0000-basic.sh

@@ -84,10 +84,6 @@ _run_sub_test_lib_test_common () {
 		passing metrics
 		'
 
-		# Tell the framework that we are self-testing to make sure
-		# it yields a stable result.
-		GIT_TEST_FRAMEWORK_SELFTEST=t &&
-
 		# Point to the t/test-lib.sh, which isn't in ../ as usual
 		. "\$TEST_DIRECTORY"/test-lib.sh
 		EOF