staging: rtl8723bs: remove code related to unsupported MCS index values

remove code related to MCS index from 8 to 31 for
rtl8723bs works only with index from 0 to 7 and
index 32.

Signed-off-by: Fabio Aiuto <fabioaiuto83@gmail.com>
Link: https://lore.kernel.org/r/1d5c8ca570a9f1880864f37099d625f96ea4a1d1.1628329348.git.fabioaiuto83@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>