Hello, It's not available in JED, there was available on a website in the past but now i'am not able to find it.
With J3.1 there is no more the content plugin Geshi. Updates don't remove the plugin. You should be able to remove it, use Extensions->Manage->Manage to uninstall it. Maybe you need to unprotect it in phpMyAdmin first. Also verify the files are gone (plugins/content/geshi).