Don't play sound for plain notification

We typically don't play a sound by default for most notifications.
However the 'notification' event for the Plasma default notifyrc
has a sound by default

Since this event is used by xdg-desktop-portal-kde, currently all
notifications from Flatpak apps have a sound by default, making
them inconsistent with their non-Flatpak counterparts.

Remove the default sound to align them.

BUG: 457672
FIXED-IN: 5.105