community/pipewire/pulseaudio: clean up -alsa "provides"

see commits

these are wrong and don't actually do anything (except allow pipewire-alsa to satisfy a pulseaudio-alsa dep), nor do they conflict with eachother. things that depend on a "pipewire audio stack" or "pulseaudio audio stack" should explicitly be depending on these if they are needed, not depends=pulseaudio-alsa and expecting one of them to be there.