To remove the mint custom search plugin (google) from Search Engine List (the bar located at top right corner), go to ‘Mange Search Engine list’ then remove ‘google’. Next, go to http://mycroft.mozdev.org/ and add a new google search engine plugin.
To remove the mint custom search from address bar search, enter ‘about:config’ in the address bar. Then enter ‘keyword.URL’ in the filter. Finally, modify its value to: