twinkle.js: Update Twinkle.addPortlet() to work after recent MW changes

Vector's dropdown menu styles were recently cleaned up and a label is
now shown by default (https://gerrit.wikimedia.org/r/#/c/112236/),
hacks that Twinkle has been doing are no longer needed and in fact
break the display (https://en.wikipedia.org/wiki/WP:VPT#Misinterface_layout).

The README is pretty confusing, I hope this is the right file to edit
(I didn't find this code in any other one).