build: Remove AUDIOD_AUDIO_FORMATS_ENUM.

build: Remove AUDIOD_AUDIO_FORMATS_ENUM.

No code actually refers to the AUDIO_FORMAT_$FOO values of this
enumeration. Its only purpose is to define the number of supported
audio formats (as the last constant of the enum). We may as well
employ ARRAY_SIZE() for that, so kill the enum and remove the loop
from configure.ac which generates it.

The related audio_formats array, which contains the names of the
supported audio formats, is only used in audiod.c, so we can remove
the declaration from audiod.h and make it static.