gdb: remove unneeded argument in check_multi_target_resumption
If we reach the modified line, resume_target is necessarily nullptr,
because of the check at the beginning of the function. So we'll
necessarily iterate on all non-exited inferiors (across all targets),
which is what we want. So just remove the unnecessary argument.
gdb/ChangeLog:
* infrun.c (check_multi_target_resumption): Remove argument to
all_non_exited_inferiors.