staging: rtl8188eu: remove code that is valid only for 5 GHz

Remove code that is used only for 5 GHz.
This addresses the below TODO item:

- find and remove remaining code valid only for 5 GHz. Most of the obvious
  ones have been removed, but things like channel > 14 still exist.

Signed-off-by: Robert Węcławski <r.weclawski@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>