staging: r8188eu: assoc_rsp and assoc_rsp_len are not used

The assoc_rsp buffer in struct mlme_priv is not used. There's no need to
store an incoming association response after receiving an processing it.

Remove the buffer pointer and its length from struct mlme_priv.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220515155401.352742-6-martin@kaiser.cx
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>