Remove unused field clone.

core/field.js

@@ -63,17 +63,6 @@ Blockly.Field.prototype.visible_ = true;
  */
 Blockly.Field.prototype.changeHandler_ = null;
 
-/**
- * Clone this Field.  This must be implemented by all classes derived from
- * Field.  Since this class should not be instantiated, calling this method
- * throws an exception.
- * @throws {goog.assert.AssertionError}
- */
-Blockly.Field.prototype.clone = function() {
-  goog.asserts.fail('There should never be an instance of Field, ' +
-      'only its derived classes.');
-};
-
 /**
  * Non-breaking space.
  */