[Lock screen] Always show "Switch User" button regardless of whether fake...

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

Summary:
Because the fake {nav 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 {nav 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 {nav Switch User}
button when session creation/switching is possible.

Test Plan: No change; {nav 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