Don't give InterruptSession special treatment

Revert "Override ck check if the last active user requested to shutdown
the session." and matching check in systemd support.

Allowing InterruptSession when another consolekit/loginctl session is
Active means that PowerDevil may attempt to autosuspend the system from
the Inactive session. Normally polkit disallows this and no harm is done
but some systems have a "ResultInactive=auth_admin" policy. This will
bring up a root dialogue on the inactive session, which is annoying and
unnecessary at best. Due to a (upower?) bug this will also block
upower suspend actions in the active session.

This reverts commit 2606ff00. It's not
clear whether https://bugs.kde.org/show_bug.cgi?id=277774 was ever fixed
by that commit.

CCBUG: 298712
REVIEW: 107022