Only check for legacy system tray icons if we're going to make one
Summary:
On Plasma is a rather pointless check as our QPT uses SNIs
so we end up checking something we've already checked,
only in a blocking way - and we're going to no-op anyway.
BUG: 385371
Test Plan:
killed kded5
created an SNI. It failed into the else branch
Reviewers: Plasma
Subscribers: plasma-devel, Frameworks
Tags: Plasma, Frameworks
Differential Revision: https://phabricator.kde.org/D8321