Remove single_channel_output from the audio API layer

[jepler]

This was added originally because it was needed on the first chips that had stereo output (samd51). However, of all the audio HW we support, this is only required for samd51 analog AudioOut, and only in certain cases (8-bit stereo or swapped 16-bit stereo). Supporting single_channel_output has been a hassle that is spread across all audio sources.

Instead, move the handling down into samd51 in a way that doesn't require support in each audio source, or have samd51 expand & channel swap into a temporary buffer as necessary.

Prompted by comments in #9876 and my own experiences with synthio & mp3decoder.

[jepler]

This WIP branch saves just shy of 1kB on feather rp2350: https://github.com/jepler/circuitpython/tree/audioio-remove-single-buffer -- that gives an idea of the best possible savings. other platforms will save less. It also removes a net ~182 lines of code.