Firefox
Visit https://addons.mozilla.org/firefox/addon/languagetool/ and click the "Remove" button:
Chrome
Visit https://chrome.google.com/webstore/detail/grammar-and-spell-checker/oldceeleldhonbafppcapldpdifcinji and click the "Remove from Chrome" button:
Opera
First, visit the extension page:
Then, click on LanguageTool:
Finally, select "Remove Extension" at the bottom of the page: