staging: brcm80211: remove wl_ops_set_rts_threshold

The function is not required as the driver does honour the USE_RTS_CTS
flag. The mac80211 API notes state that either one or the other is required
(see http://wireless.kernel.org/en/developers/Documentation/mac80211/API

).

Cc: devel@linuxdriverproject.org
Cc: linux-wireless@vger.kernel.org
Cc: Brett Rudley <brudley@broadcom.com>
Cc: Henry Ptasinski <henryp@broadcom.com>
Cc: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>