The Boards Manager provides the most convenient way to remove unwanted or unused boards. The location of the installed cores can be found here.
The following instructions to uninstall is not limited to the core in this example; it equally would be applicable to both official Arduino and third party cores.
Open Boards Managers in the Arduino IDE menu in
Tools > Board > Boards Manager.
Once the Boards Manager is open, search for the name of the core you want to uninstall.
Find the correct core in the search results and proceed by clicking the
Ensure that the correct core has been selected in the text box and confirm to uninstall by clicking the "Yes" button.
Close the Boards Manager