Remove unsupported X sessions

In cfdcdfb we removed the "Custom" session from the greeter but the
"custom" bit was left here. Remove it because it has no use anymore.

Also remove the "default" session that runs startkde as it makes no
sense.
Actually tell daemon the authentication has failed

Send a message back to the daemon when the authentication fails, this
way the greeter will display an error instead of being restarted as
soon as sddm-helper finished.

Closes #221
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