ARM: at91: remove unused duplicated filechk_offsets

The filechk_offsets in arch/arm/mach-at91/Makefile is never
used because it is always overridden by the equivalent one in
scripts/Makefile.lib

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>