Drop support for GTH board

The board maintainer states:

    The GTH board is obsolete and has not been manufactured for
    several years.
    To my knowledge, no recent U-Boot build has been tested on that
    card.

So drop support for this board.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Thomas Lange <thomas@corelatus.se>
Acked-by: Thomas <Lange&lt;thomas@corelatus.se>