[applets/taskmanager] Don't show tooltip subtitle that's identical to the main text

Summary: Before-and-after pictures below should be self-explanatory:

Test Plan:
Before: {F8094802}
After: {F8094803}

Reviewers: #vdg, #plasma, filipf

Reviewed By: #vdg, filipf

Subscribers: filipf, plasma-devel

Tags: #plasma

Maniphest Tasks: T12640

Differential Revision: https://phabricator.kde.org/D27273