Remove button to move to the system sound settings

This pointed to (what is now) phononsettings before and currently points to the kcm from plasma-pa. While the settings obviously might affect Amarok, the configure dialog is about configuring Amarok itself and I wouldn't expect a button to change the settings for my network configuration either, for example. Furthermore this avoids an optional runtime dependency on pulseaudio and plasma-pa, which is probably only available on Linux/*BSD.