brcmsmac: main: Eliminate empty brcms_c_down_del_timer()

This function does nothing so remove it. This addresses the following
coccicheck warning:

drivers/net/wireless/broadcom/brcm80211/brcmsmac/main.c:5103:6-15:
Unneeded variable: "callbacks". Return "0" on line 5105

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200910140446.1168049-1-yanaijie@huawei.com
Link: https://lore.kernel.org/r/20200910140455.1168174-1-yanaijie@huawei.com