powerpc: drop unused GENERIC_CSUM Kconfig item

Commit d4fde568 ("powerpc/64: Use optimized checksum routines on
little-endian") converted last powerpc user of GENERIC_CSUM.

This patch does a final cleanup dropping the Kconfig GENERIC_CSUM
option which is always 'n', and associated piece of code in
asm/checksum.h

Fixes: d4fde568

 ("powerpc/64: Use optimized checksum routines on little-endian")
Reported-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>