wifi: mac80211: Remove unused function declarations

Commit 68542962 ("mac80211: Fix circular locking dependency in
ARP filter handling") left the ieee80211_set_arp_filter() declaration
unused.
And commit 164eb02d ("mac80211: add radar detection command/event")
introducted ieee80211_dfs_cac_timer() declaration but never implemented
it.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20230801134337.24452-1-yuehaibing@huawei.com


[reword commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>