run-command API: move *_tr2() users to "run_processes_parallel()"

run-command API: move *_tr2() users to "run_processes_parallel()"

Have the users of the "run_processes_parallel_tr2()" function use
"run_processes_parallel()" instead. In preceding commits the latter
was refactored to take a "struct run_process_parallel_opts" argument,
since the only reason for "run_processes_parallel_tr2()" to exist was
to take arguments that are now a part of that struct we can do away
with it.

See ee4512ed481 (trace2: create new combined trace facility,
2019-02-22) for the addition of the "*_tr2()" variant of the function,
it was used by every caller except "t/helper/test-run-command.c"..

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