audiomixer: systemd: depend on pipewire instead of wireplumber

audiomixer: systemd: depend on pipewire instead of wireplumber

Depending on wireplumber.service is broken when wireplumber@ template
services are used. It is also not needed now that ensure_controls() does
not block waiting for nodes to appear. The service will gracefully start
as long as it can connect to pipewire, so it needs to depend on pipewire
instead.

Bug-AGL: SPEC-4934
Signed-off-by: George Kiagiadakis <george.kiagiadakis@collabora.com>
Change-Id: I32fe5f186252392a0b62006c1f9da3837cbc97e7