run-command API: have "run_processes_parallel{,_tr2}()" return void

run-command API: have "run_processes_parallel{,_tr2}()" return void

Change the "run_processes_parallel{,_tr2}()" functions to return void,
instead of int. Ever since c553c72eed6 (run-command: add an
asynchronous parallel child processor, 2015-12-15) they have
unconditionally returned 0.

To get a "real" return value out of this function the caller needs to
get it via the "task_finished_fn" callback, see the example in hook.c
added in 96e7225b310 (hook: add 'run' subcommand, 2021-12-22).

So the "result = " and "if (!result)" code added to "builtin/fetch.c"
d54dea77dba (fetch: let --jobs=<n> parallelize --multiple, too,
2019-10-05) has always been redundant, we always took that "if"
path. Likewise the "ret =" in "t/helper/test-run-command.c" added in
be5d88e1128 (test-tool run-command: learn to run (parts of) the
testsuite, 2019-10-04) wasn't used, instead we got the return value
from the "if (suite.failed.nr > 0)" block seen in the context.

Subsequent commits will alter this API interface, getting rid of this
always-zero return value makes it easier to understand those changes.

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