in developer and beta you can do it from start page settings cog
in stable, this option is not landed yet, so it requires entering Konami code on settings page
after you enter it, you will have option to remove google search from speed dial
more info how to enter Konami code
http://techdows.com/2014/06/use-konami-code-to-enable-power-user-settings-in-opera.html