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

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>