remote: Move discard_pending_stop_replies call

remote: Move discard_pending_stop_replies call

This helps because discard_pending_stop_replies will later become a
method of remote_target.  Otherwise, when we have multiple instances
of remote_target, we'd have to make discard_pending_stop_replies find
the inferior's target_ops, and upcast it to remote_target (if indeed a
remote) to find the pending stop replies queue to clear.

gdb/ChangeLog:
2018-05-22  Pedro Alves  <palves@redhat.com>

	* remote.c (remote_target::mourn_inferior): Move
	discard_pending_stop_replies call here from ...
	(_initialize_remote): ... here.