gdb: remove use of iterate_over_inferiors in mi/mi-interp.c

Replace it with a range-based for.  I've updated the comment in
mi_interp::init, which was a bit stale.

gdb/ChangeLog:

	* mi/mi-interp.c (report_initial_inferior): Remove.
	(mi_interp::init): Use range-based for to iterate over inferiors.