Revert "test-lib: whitelist GIT_TR2_* in the environment"

This reverts my commit c1ee5796dc ("test-lib: whitelist GIT_TR2_* in
the environment", 2019-03-30), which is now redundant.

Since e4b75d6a1d ("trace2: rename environment variables to
GIT_TRACE2*", 2019-05-19) the GIT_TRACE2* variables match the existing
GIT_TRACE* pattern added in 95a1d12e9b ("tests: scrub environment of
GIT_* variables", 2011-03-15), so we no longer need to list TR2 here.

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

t/test-lib.sh

@@ -386,7 +386,6 @@ unset VISUAL EMAIL LANGUAGE COLUMNS $("$PERL_PATH" -e '
 	my @env = keys %ENV;
 	my $ok = join("|", qw(
 		TRACE
-		TR2_
 		DEBUG
 		TEST
 		.*_TEST