If you are not able to get an option to uninstall then you will need to quit Safari and manually remove the third party add on installed on your computer.
Code:
/Library/Internet Plug-Ins/
/Library/Input Methods/
/Library/InputManagers/
/Library/ScriptingAdditions
The above mentioned four location are listed in root-level Library which located on hard disk and not user-level Library located at the Home folder.
Code:
~/Library/Internet Plug-Ins/
~/Library/Input Methods/
~/Library/InputManagers/
~/Library/ScriptingAdditions
tilde (~) represents the Home folder on the above mentioned location.
Bookmarks