You can't delete this group. However, if it is no longer showing up, then it is possible all of the devices within it were deleted. If that was the case, the only way to recover would be to restore from a known good backup, or to manually re-add the missing devices.
If the LMS server is integrated with ACS, it is also possible that all of the missing devices were removed from the ACS server, and are now showing up under Common Services > Device and Credentials > Reports > Devices not in ACS.