Staging: rtl8723au: Remove unused functions

The functions rtw_get_oper_bw23a and rtw_get_oper_ch23aoffset are never
used anywhere in the kernel. So, remove their definition and prototype.
Grepped to find occurences.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Reviewed-by: Julian Calaby <julian.calaby@gmail.com>
Acked-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>