wifi: rtl8xxxu: remove obsolete priv->vif

Now that all uses of priv->vif have been converted to priv->vifs[]
remove the old attribute.

Signed-off-by: Martin Kaistra <martin.kaistra@linutronix.de>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20231222101442.626837-19-martin.kaistra@linutronix.de