ARM: at91: remove board file for Acme Systems Fox G20

As Acme Systems Fox G20 is available in Device Tree flavor and that we plan to
remove all the board files soon, we can remove this one without problem.
If you use this board, please use a DT-enabled at91sam9g20 kernel with
at91-foxg20.dts.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Sergio Tanzilli <tanzilli@acmesystems.it>