login: don't remove all devices from PID1 when only one was removed

login: don't remove all devices from PID1 when only one was removed

FDSTOREREMOVE=1 removes all fds with the specified name.  And we had named
the fds after the session.  Better fix that.

Closes #8344.

AFAICT there's no point providing compatibility code for this transition.
No-one would be restarting logind on a system with a GUI (where the
session devices are used), because doing so has been killing the GUI, and
even causing startup of the GUI to fail leading to a restart loop.

Upgrading logind on a running system with a GUI might start being possible
after this commit (and after also fixing the display server of your
choice).