Remove phonon KCM

Summary:
Most of the Phonon KCM settings have been moved to plasma-pa. The remaining settings server very specific use cases that are mostl likely not worth exposing them front and center.

@sitter said he has a version of the settings to be integtated into phonon so the remaining three users can still access them

See T9091

Test Plan: builds

Reviewers: sitter

Reviewed By: sitter

Subscribers: kde-doc-english, ngraham, pino, plasma-devel, sitter

Tags: Plasma, Documentation

Differential Revision: https://phabricator.kde.org/D22616