If you want to stop using the Google Email service, all you need to do is remove the custom records added in the MX Entries for your domain name. Again this is performed via cPanel -> Mail section -> MX Entry. You just need to click the delete link and then confirm by clicking the Delete button in the pop-up.

Note that you should delete all Google mail records following those instructions in order to avoid any discrepancies. Note also that is important in such case to add again the default MX record for your domain name. You can use priority 0 and and your domain name as destination.