Once the button 'Add Extra Variable' is clicked it is impossible to remove the added form for new extra var.
Can you provide a screenshot of the "Extra Variables" section of your build job and which browser(s) you've tried? I've tried Chrome and Firefox on both Linux and Windows and see a red X to remove any extra vars I add in all cases.
Closing as cannot reproduce since neither Michael Cresswell nor me can reproduce it anymore. (Tested on Linux, latest weekly Jenkins 2.104, and ansible-plugin 0.8 in Google Chrome)