staging: rtl8723bs: remove rtw_br_ext.h

The header rtw_br_ext.h is not used anywhere.
'git grep rtw_br_ext.h' returns nothing, remove the file.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>