audio: simplify implementation of property ao-volume

ao-volume is represented in the code with a `struct ao_control_vol_t`
which contains volumes for two channels, left and right.

However the code implementing this property in command.c never treats
these values individually. They are always averaged together.
On the other hand the code in the AOs handling these values also has to
handle the case where *not* exactly two channels are handled.

So let's remove the `struct ao_control_vol_t` and replace it with a
simple float.
This makes the semantics clear to AO authors and allows us to drop some code from the AOs and command.c.

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