rtl8723au: Remove code dependent on CONFIG_CONCURRENT_MODE

As only the RTL8192DU has a multiple MAC, there is no need for concurrent
devices here.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
rtl8723au: Remove code dependent on CONFIG_TX_MCAST2UNI

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
rtl8723au: Remove dead code with CONFIG_DUALMAC_CONCURRENT

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
rtl8723au: Remove CONFIG_DISABLE_MCS13TO15

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>