You often can't remove the Google search bar on stock launchers. Try long-pressing it to see if any options come up to remove it from the screen. You can also try long-pressing any empty part of the homescreen, then select the option for homescreen settings. See if there's an option there to turn the search bar on or off.
If none of this works, then you can't remove it. You'd have to install a 3rd party launcher, like Nova, which is much more customizable.