doc-rst: auto-generate audio.h.rst

This is an auto-generated header. Remove the hardcoded one
and do the right thing here.

NOTE: this is a deprecated API. So, we won't make any
effort to try identifying the meaning of this obscure
API that is used only on a legacy driver.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>