Remove iterate_over_inferiors

Remove iterate_over_inferiors

The last caller of iterate_over_inferiors is darwin-nat.c.  This patch
removes the calls from this file, and then remove
iterate_over_inferiors.

In general I think "external iteration" is to be preferred in gdb, the
main benefit being that the code is easier to read.

I rebuilt this on Darwin.  I seem to only have access to Darwin
systems where gdb does not yet work :-(, so I can't run the test
suite.

gdb/ChangeLog
2020-04-23  Tom Tromey  <tom@tromey.com>

	* inferior.h (iterate_over_inferiors): Don't declare.
	* inferior.c (iterate_over_inferiors): Remove.
	* darwin-nat.c (find_inferior_task_it, find_inferior_pid_it):
	Remove.
	(darwin_find_inferior_by_task, darwin_find_inferior_by_pid): Don't
	use iterate_over_inferiors.
	(darwin_resume_inferior_it)
	(struct resume_inferior_threads_param)
	(darwin_resume_inferior_threads_it): Remove.
	(darwin_nat_target::resume): Don't use iterate_over_inferiors.

Change-Id: Ib2fdf2c98e40f13156ff869ed3173d5f1fdae7ea