ath9k: Remove aggregation flags

SC_OP_TXAGGR and SC_OP_RXAGGR are not really needed.
The HT capabilities of the station and HW can be used instead.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>