main/postmarketos-ui-phosh: re-enable autologin

We have tried out lightdm-mobile-greeter for three weeks, but let's remove
it again from the default install.

The greeter behaves completely different from the Phosh (or any other
UI's) lockscreen, therefore it provides a bad user experience. Not only
the design is different, but it also can't display the same information
as the Phosh lockscreen. You can't accept a call coming in while the
greeter is displayed, and whatever other features will be added to the
lockscreen in the future would not be in the greeter (e.g. making a photo
without logging in, emergency calls without login, ...).

It's clear to me now, that postmarketOS in the default installation
should install one UI in single-user mode, and that UI should use its
lockscreen to display all login screens. The case of having multiple UIs
installed at the same time is used rarely, so instead of optimizing for
it, document how to enable a greeter after the installation in the wiki:
https://wiki.postmarketos.org/wiki/Configure_postmarketOS_for_multiple_UIs_or_users

Besides the strong reasons above to ship with no greeter at all, we had
problems with lightdm-mobile-greeter in particular:
* It did not work with Plasma Mobile
* When HDMI was plugged in, the greeter was not usable (especially
  annoying, because Phosh tends to crash after plugging in HDMI, so it
  would go back to the greeter which then means you need to reboot)

This reverts commit be693339.

I've verified with qemu that this boots into phosh with autologin again.

Leading up to this MR, I did a lot of soul searching related to display managers in postmarketOS (how can we get rid of setting the username in post-install? how do we display phosh lockscreen on boot?) and I think I found a good path forward in #823 (closed) (let's discuss that there).

• Fixes: #789 (closed), #794 (closed)
• Closes: #320 (closed)