[Lock screen] Always show "Switch User" button regardless of whether fake "switch user" item is shown

Summary:
Because the fake Switch User item in the switcher UI counts as a session, the button
was always being shown. But if and when that changes and the fake item is no longer shown,
the Switch User button itself will stops being shown potentially locking out
non-logged-in users in multi-user computers where the only logged-in user has locked the
screen.

This patch drops the conditional check entirely and always shows the Switch User
button when session creation/switching is possible.

Test Plan: No change; Switch User button is still shown.

Reviewers: mart, Plasma, VDG, filipf

Reviewed By: VDG, filipf

Subscribers: filipf, davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D23279