ARM: at91: remove two unused headers

Commit 82c583e3 ("AT91RM9200 hardware
headers") introduced arch/arm/mach-at91/include/mach/at91_spi.h and
arch/arm/mach-at91/include/mach/at91_ssc.h. (These files were called
at91rm9200_spi.h and at91rm9200_ssc.h then.) That commit was added in
the v2.6.18 development cycle.

Nothing includes them now and nothing uses the named constants they
provide. (There's no way these named constants could be used unless
these files were included somehow.) It actually seems these two headers
have never been used since they were added to the tree. They can safely
be removed.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>