mac80211: Remove redundant preamble and RTS flag setup in minstrel_ht

mac80211 does the same afterwards anyway. Hence, just drop
this redundant code.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>