kde6.xml remove "audio/x-pn-realaudio-plugin"

Reasons:
This is not a file type. This is an implementation detail
of the British Broadcasting Corporation (BBC) web site in order
or the correct RealPlayer web browser
NPAPI plugin to get loaded.
It is an implementation detail of the BBC web page before they
switched to HTML5 and before they switched to Flash Player.
RealPlayer still exists but does not provide a Netscape Plugin API
web browser plugin anymore.
BBC web sites stopped serving audio and video content as real player more
than 10 years ago.