ath10k: remove pci completion list

One of the premises was to guarantee serialized
completion handling for upper layers
(HTC/WMI/HTT). Since quite some time now it is no
longer necessary.

The other premise was to batch up tx/rx
completions to take advantage of hot caches.
However frame tx/rx completion indications come in
on a single pipe already so they are already
batched up. More meaningful batching is done in
HTT itself.

This means PCI completion is no longer necessary
to keep around. It just wastes memory, cycles and
SLOC.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>