Can't remove groups from WebAdmin

[madmaxoft]

Reported by @Shadowraix in cuberite/cuberite#2083:

If you try to remove a group within the ranks tab under 'edit groups' in WebAdmin it returns this error.

"An internal error has occurred, no handler for subpage removegroup."

[HelenaKitty]

I have mentioned this problem to @sphinxc0re on his forum thread libcuberite as I strongly believe any new administration interfaces should be put on hold until the webadmin panel is fully functional, You can find the thread at https://forum.cuberite.org/thread-2740.html

I really do believe this is a major, but simple issue when it comes down to administering Cuberite servers.