tries to start nonexistant sessions if the user last used them

## Prerequisites
- [x] I have searched open and closed issues for duplicates.

## Describe the bug
https://github.com/elementary/greeter/pull/290 makes it so we use the latest session from the user. However it's possible that the latest session from the user was removed after some transaction.

Here's a illustration of how that situation can happen:
1. User has Pantheon and Gnome 3 on their system.
(or any other environment)
2. User logs into Gnome3
3. User removes Gnome3
4. User restarts
5. User starts Pantheon's greeter
6. Try to login but the greeter is trying the old Gnome 3 session.

And the transaction at 2-4 can vary. All that matters is they logged into a session before and somehow it was removed at some point before the greeter is started.

A workaround for this could be manually selecting the newly desired session in the chooser, but if there's only one session it doesn't appear for you to interact with it.

And just like https://github.com/elementary/greeter/issues/212, this is pretty problematic for other linux distro's.