b43: remove set but not used variable 'wl'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/wireless/broadcom/b43/main.c: In function 'b43_one_core_detach':
drivers/net/wireless/broadcom/b43/main.c:5496:17: warning:
 variable 'wl' set but not used [-Wunused-but-set-variable]

After commit 644aa4d6 ("b43: remove list of IEEE 802.11 devices")
'wl' is not used any more.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>