Remove the deprecated destroy method of the API

This is deprecated since October 2015 with a visible message, so we can
safely remove this now.

src/display/api.js

@@ -1063,14 +1063,6 @@ var PDFPageProxy = (function PDFPageProxyClosure() {
       return Promise.all(waitOn);
     },
 
-    /**
-     * Cleans up resources allocated by the page. (deprecated)
-     */
-    destroy() {
-      deprecated('page destroy method, use cleanup() instead');
-      this.cleanup();
-    },
-
     /**
      * Cleans up resources allocated by the page.
      */