Removed use of normalize().

According to
https://github.com/ckeditor/ckeditor5-engine/issues/576#issuecomment-242041069.

tests/model/composer/modifyselection.js

@@ -387,16 +387,6 @@ describe( 'Delete utils', () => {
 
 	function test( title, input, output, options ) {
 		it( title, () => {
-			// FIXME: This is wrong. Proper polyfill should be used, like https://github.com/walling/unorm.
-			if ( !String.prototype.normalize ) {
-				String.prototype.normalize = function() {
-					return String( this );
-				};
-			}
-
-			input = input.normalize();
-			output = output.normalize();
-
 			setData( document, input );
 
 			// Creating new instance of selection instead of operation on engine.model.Document#selection.