wifi: wilc1000: Remove unused declarations

Commit 8399918f ("staging: wilc1000: use RCU list to maintain vif interfaces list")
removed wilc_get_interface() but not its declaration.
Commit 9bc061e8 ("staging: wilc1000: added support to dynamically add/remove interfaces")
declared but never implemented wilc_cfg_alloc() and wilc_netdev_interface().

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230819102100.13720-1-yuehaibing@huawei.com