gdb: make record-full clear async handler in wait

gdb: make record-full clear async handler in wait

For the same reason explained in the previous patch (which was for the
record-btrace target), move clearing of the async event handler of the
record-full target to the wait method.

I'm not sure if/where that target needs to re-set its async event
handler in the wait method.  Since it only supports a single thread,
there probably can't be multiple events to report at the same time.

gdb/ChangeLog:

	* record-full.c (record_full_async_inferior_event_handler):
	Don't clear async event handler.
	(record_full_base_target::wait): Clear async event handler at
	beginning.

Change-Id: I146fbdb53d99e3a32766ac7cd337ac5ed7fd9adf