ath10k: remove set but not used variable 'num_tdls_vifs'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/wireless/ath/ath10k/mac.c: In function 'ath10k_sta_state':
drivers/net/wireless/ath/ath10k/mac.c:6238:7: warning:
 variable 'num_tdls_vifs' set but not used [-Wunused-but-set-variable]

'num_tdls_vifs' not used any more after
  9a993cc1 ("ath10k: fix the logic of limiting tdls peer counts")

Also, remove the single called function ath10k_mac_tdls_vifs_count
and ath10k_mac_tdls_vifs_count_iter.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>