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