Delete redundant and monochrome versions of applications-internet

Summary:
applications-internet is the only category icon that has monochrome versions at small
sizes. In addition, there's a copy of it (same name and appearance) in preferences.

This patch corrects those two issues by deleting the monochrome and redundant versions.

Test Plan:
Only the colorful version shows up in Cuttlefish:

With the patch to add icons to Discover's sidebar (D22716), all icons are now colorful:

Reviewers: VDG, ndavis

Reviewed By: VDG, ndavis

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22733