dnd: Rename DND.removeMonitor() to DND.removeDragMonitor()

DND.addDragMonitor() and DND.removeMonitor() are inconsistently
named, so rename the latter.

https://bugzilla.gnome.org/show_bug.cgi?id=652730

js/ui/dnd.js

@@ -61,7 +61,7 @@ function addDragMonitor(monitor) {
     dragMonitors.push(monitor);
 }
 
-function removeMonitor(monitor) {
+function removeDragMonitor(monitor) {
     for (let i = 0; i < dragMonitors.length; i++)
         if (dragMonitors[i] == monitor) {
             dragMonitors.splice(i, 1);