Once the button 'Add Extra Variable' is clicked it is impossible to remove the added form for new extra var.
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)
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.