appDisplay: Remove redundant checks for pageNumber

We already do these at the beginning of the function.

https://bugzilla.gnome.org/show_bug.cgi?id=729064

js/ui/appDisplay.js

@@ -472,14 +472,12 @@ const AllView = new Lang.Class({
         // longer than PAGE_SWITCH_TIME
         time = Math.min(time, PAGE_SWITCH_TIME);
 
-        if (pageNumber < this._grid.nPages() && pageNumber >= 0) {
         this._currentPage = pageNumber;
         Tweener.addTween(this._adjustment,
                          { value: this._grid.getPageY(this._currentPage),
                            time: time,
                            transition: 'easeOutQuad' });
         this._pageIndicators.setCurrentPage(pageNumber);
-        }
     },
 
     _diffToPage: function (pageNumber) {