I was redirected here by the support department since a functionnality seems to be missing from RDM.
While we can create "Custom" Server Roles, we can not edit/rename or delete them afterward.
If you ask me, it sound more like a bug then a nice to have "new feature", but I'm following the support process.
RDM does not remember the custom server roles if it's no longer used. You could try to use the Advanced Search to find where it's used and remove it from all the sessions.