ath10k: remove unused fw_desc processing

The fw descriptor was never used and probably never will be. It makes
little sense to maintain support for it. Remove it and simplify rx
processing. This will make it easier to optimize rx processing later
as well.

Signed-off-by: Rajkumar Manoharan <rmanohar@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>