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:

{F5779980}

{F5779981}

{F5779982}

Reviewers: apol, #discover_software_store

Subscribers: rkflx, plasma-devel

Tags: #plasma

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