[ermine] Remove “add” command

One should use sessionctl add_mod instead. The entire ermine_ctl
program will be removed once sessionctl supports “list” and “remove”.

Testing: built, logged in as guest, started mods with sessionctl and
remove them with ermine_ctl “list” and “remove”

Change-Id: I3fc412fea63b8d52e723a494dc7c5aab312d2c0d
3 files changed