ao_pipewire: avoid setting any media.role

By default, WirePlumber restores the volume of each application to
whatever the last application with the same media role used. This is
completely insane behavior, and I couldn’t find any way for a user to
opt out of it on my end. The `state-stream.lua` script responsible for
this mess only uses media.role if the property is present, so the only
reliable workaround is simply not to set one at all.

This WirePlumber behavior is inherited from PulseAudio, which also
exhibits the same behavior to this day.

This commit also mirrors the same change made to ao_pulse in 2015:
b7325b2

Author: llyyr

audio/out/ao_pipewire.c

@@ -577,7 +577,6 @@ static int init(struct ao *ao)
     struct pw_properties *props = pw_properties_new(
         PW_KEY_MEDIA_TYPE, "Audio",
         PW_KEY_MEDIA_CATEGORY, "Playback",
-        PW_KEY_MEDIA_ROLE, ao->init_flags & AO_INIT_MEDIA_ROLE_MUSIC ?  "Music" : "Movie",
         PW_KEY_NODE_NAME, ao->client_name,
         PW_KEY_NODE_DESCRIPTION, ao->client_name,
         PW_KEY_APP_NAME, ao->client_name,