tests: drop GIT_*_TIMING_TESTS environment variable support

Two tests (t3302 and t3419) used to have their own environment
variable to trigger expensive tests without enabling expensive
tests in other scripts; a user could set GIT_NOTES_TIMING_TESTS
but not GIT_TEST_LONG and run the whole test suite and trigger
expensive tests only in t3302 but not other tests.  The same for
GIT_PATCHID_TIMING_TESTS in t3419.

While this may have seemed a good flexibility, in reality if you are
concentrating on a single test (e.g. t3302), you can just run that
single test with the GIT_TEST_LONG to trigger expensive tests.  It
does not seem worth forcing other people who may want to come up
with their own expensive tests to invent new environment variables
by keeping this convention.

Drop them.

Signed-off-by: Junio C Hamano <gitster@pobox.com>