Stop ungrabbing when freeing XI devices

This breaks when x_free_xi_devices is called from a
HierarchyChange event where a master device has been removed or
a slave device has been detached, and it should no longer be
required since we now rely on passive grabs.

* src/xterm.c (x_init_master_valuators): Stop calling
XIUngrabDevice on all devices.

src/xterm.c

@@ -378,8 +378,6 @@ x_free_xi_devices (struct x_display_info *dpyinfo)
     {
       for (int i = 0; i < dpyinfo->num_devices; ++i)
 	{
-	  XIUngrabDevice (dpyinfo->display, dpyinfo->devices[i].device_id,
-			  CurrentTime);
 	  xfree (dpyinfo->devices[i].valuators);
 
 	  tem = dpyinfo->devices[i].touchpoints;