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