Fix for issue 2073.  Get rid of the swap button and swapText() function.

With more than two fields, pressing the “swap” button in the editor oddly [cleared the second](https://code.google.com/p/ankidroid/issues/detail?id=2073) field.

As [nobody defended](https://groups.google.com/forum/#!topic/anki-android/4POz1Bu7Q8k) keeping the button for two-field models, just cut it out.