ath11k: Remove unnecessary delay in ath11k_core_suspend

The intended delay in ath11k_core_suspend is introduced in commit
d1b0c338 ("ath11k: implement suspend for QCA6390 PCI devices"),
now with ath11k_mac_wait_tx_complete added in commit ba9177fc
("ath11k: Add basic WoW functionalities"), that delay is not
necessary now, so remove it.

This is found in code review.

Tested-on: WCN6855 hw2.0 PCI WLAN.HSP.1.1-02431-QCAHSPSWPL_V1_V2_SILICONZ_LITE-1

Signed-off-by: Baochen Qiang <quic_bqiang@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220331002105.1162099-1-quic_bqiang@quicinc.com