Add command to remove environment from config file

[dvaccarosenna]

Is your feature request related to a problem? Please describe.
Users can mostly manage their LMCTL config file without having to open the file in an editor. New environments can be added with lmctl login and the active environment can be changed with lmctl use env.
However, there's no command to remove an environment. This requires finding and modifying the config file.

Describe the solution you'd like
Introduce lmctl delete env command.
Should allow a user to delete the active environment as well.