KDE: change default sound mixer applet

The kmix mixer is being replaced by the more flexible and integrated
plasma-pa mixer from audio/plasma5-plasma-pa

For this:

	- remove audio/kmix from the default options set in the metaport
	  multimedia/kdemultimedia

	- add audio/plasma5-plasma-pa to the default options set in the
	  metaport x11/plasma5-plasma

If you do not want pulseaudio on your system and want to keep using
the old mixer, you have multiple options:

	a) don't install the metaports (they only pull in other
	   packages)
	b) manually build multimedia/kdemultimedia and
	   x11/plasma5-plasma with your chosen options
	c) install both

Reported by:		Graham Perrin <grahamperrin@gmail.com>
PR:			258579