ath11k: remove duplicate function declaration

Removed the duplicated peer related function declaration
from core.h since those functions are already declared in peer.h

Founded in code review.

Tested-on: IPQ8074 hw2.0 AHB WLAN.HK.2.4.0.1-01492-QCAHKSWPL_SILICONZ-1

Signed-off-by: Karthikeyan Periyasamy <periyasa@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1608304793-20612-1-git-send-email-periyasa@codeaurora.org