afs: Simplify open_next_audio_file().

afs: Simplify open_next_audio_file().

This function of afs.c is called whenever the virtual streaming
system requests a new audio file. It determines the row of the audio
file table that corresponds to the audio file with highest score,
according to the current mood or playlist. The function passes this
information to open_and_update_audio_file() of aft.c which sets up
an afd structure to be passed back to the server process.

Letting open_and_update_audio_file() determine which file to
open is shorter, and it allows to get rid of two arguments to
open_and_update_audio_file().