lavfi/audio: eliminate ff_default_filter_samples().

lavfi/audio: eliminate ff_default_filter_samples().

It currently does the following:
1) get a zeroed audio buffer
2) copy some properties (but not the data) of the input buffer to it
3) pass this buffer to the output filter
This looks useless and is indeed not used by any filters, therefore
delete it.

Make ff_null_filter_samples() (just pass the buffer to the next filter)
the new default.