main/postmarketos-ui-*: do not hardcode pulseaudio

pmos-base-ui depends on pmos-base-ui-audio, so some audio backend should be installed for all UIs. Hardcoding pulseaudio in UI packages prevents users from switching to pipewire, so let's remove that. As of today the default is still pulseaudio for the backend, so there should be no functional change for these UIs by removing the hardcoded dep on PA.