nl80211: Keep optional check for KCK in set rekey

In case of new AKM suites (example FILS-SHA256), KCK is
optional in rekey data.
commit 5dedc767 ("cfg80211: Include length of kek in
rekey data") brings in the change to make KCK optional but
does not remove the kernel change already present to always
check for KCK in set rekey data.

This commit addresses the same by removing the check.

CRs-Fixed: 2469060
Change-Id: Iac8292c973b5562bde33ccc8a054a7a06927d517
Signed-off-by: Ankita Bajaj <bankita@codeaurora.org>