Kill supported_audio_formats().

It's kind of silly to have a function that just returns a constant string
which is known at compile time. Even more silly is to contruct this string
using an #ifdef for each optional audio format when we already
constructed the space-separated list of supported audio formats during
configure.

So just use AC_DEFINE_UNQUOTED() in configure.ac to make the
list of supported audio formats available via config.h and get rid of
supported_audio_formats(). Rename it to SERVER_AUDIO_FORMATS
and kill a useless space character while we're at it.