Remove categories from app delegates

Summary:
This patch removes category labels from the app delegates, for the following reasons:

• They're not very useful in the first place; the caption generally provides all the information you need without click on it for a more detailed view
• They clutter up the presentation and make it harder to parse quickly
• For certain apps and certain views, they sometimes don't show up at all, which is confusing and inconsistent

Test Plan:
Looks better, and it's easier to quickly parse the list:

Reviewers: apol, Discover Software Store

Subscribers: rkflx, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D11842