Use delete instead of xfree for gdb_timer

gdb_timer objects are new'ed in create_timer, but xfree'd in
poll_timers.  Use delete instead.

gdb/ChangeLog:

	* event-loop.c (poll_timers): Unallocate timer using delete
	instead of xfree.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2017-06-02  Simon Marchi  <simon.marchi@ericsson.com>
+
+	* event-loop.c (poll_timers): Unallocate timer using delete
+	instead of xfree.
+
 2017-06-02  Simon Marchi  <simon.marchi@polymtl.ca>
 
 	* breakpoint.h (struct breakpoint_ops) <dtor>: Remove.

gdb/event-loop.c

@@ -1270,7 +1270,7 @@ poll_timers (void)
       /* Delete the timer before calling the callback, not after, in
 	 case the callback itself decides to try deleting the timer
 	 too.  */
-      xfree (timer_ptr);
+      delete timer_ptr;
 
       /* Call the procedure associated with that timer.  */
       (proc) (client_data);