audio: fix previous commit

This would have always forced mono first (if supported by the AO),
instead of stereo.
ao_alsa: remove some dead code

This was only added recently (c1e97161) as an attempt to minimize the
bad impact of channel layout device aliases. But use of these was
removed in commit 49df0132. Now this code does pretty much nothing, and
shouldn't be needed anymore. It does something when using spdif, but
this fallback won't work anyway.