gdb: remove uses of iterate_over_inferiors in top.c

Replace with range-based for loops.

gdb/ChangeLog:

	* top.c (struct qt_args): Remove.
	(kill_or_detach): Change return type to void, replace `void *`
	parameter with a proper one.
	(print_inferior_quit_action):  Likewise.
	(quit_confirm): Use range-based for loop to iterate over inferiors.
	(quit_force): Likewise.