ath11k: remove unused tx ring counters

remove unused counter to avoid taking locks inorder to optimize
cpu load.

Using Flamegraph, cpu usage of ath11k_dp_tx() observed to be decreased
from 5.58% to 3.74% with iperf traffic running with 80MHz bandwidth ap
mode.

Signed-off-by: Pradeep Kumar Chitrapu <pradeepc@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>