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.

Change-Id: If95704915dca19599d5f7f4732bbd6ccd20bf6b4