gdb: make record-btrace clear event handler in wait

gdb: make record-btrace clear event handler in wait

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

The record-btrace target already re-sets its async event handler in its
wait method, so that part doesn't need to be changed:

    /* In async mode, we need to announce further events.  */
    if (target_is_async_p ())
      record_btrace_maybe_mark_async_event (moving, no_history);

gdb/ChangeLog:

	* record-btrace.c (record_btrace_handle_async_inferior_event):
	Don't clear async event handler.
	(record_btrace_target::wait): Clear async event handler at
	beginning.

Change-Id: Ib32087a81bf94f1b884a938c8167ac8bbe09e362