rt2x00: remove redundant functions rt2x00mac_sta_{add/remove}

Only rt2800 subdriver of rt2x00 implement sta_add() and sta_remove(),
rt2x00mac_sta_add and rt2x00mac_sta_remove has no callers after
commit 9c87758c

 ("rt2x00: call sta_add/remove directly in rt2800").
So can be removed.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>