Move quadKey function to the one place it is used

test/spec/ol/tilecoord.test.js

@@ -1,5 +1,4 @@
 import {
-  quadKey,
   getKey,
   fromKey,
   hash,
@@ -19,14 +18,6 @@ describe('ol.TileCoord', function() {
     });
   });
 
-  describe('call quadKey', function() {
-    it('returns expected string', function() {
-      const tileCoord = [3, 3, 5];
-      const s = quadKey(tileCoord);
-      expect(s).to.eql('213');
-    });
-  });
-
   describe('getKey()', function() {
     it('returns a key for a tile coord', function() {
       const key = getKey([1, 2, 3]);