powerpc: drop unused GENERIC_CSUM Kconfig item

Commit d4fde568a3 ("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: d4fde568a3 ("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>

arch/powerpc/Kconfig

@@ -247,9 +247,6 @@ config PPC_BARRIER_NOSPEC
     default y
     depends on PPC_BOOK3S_64 || PPC_FSL_BOOK3E
 
-config GENERIC_CSUM
-	def_bool n
-
 config EARLY_PRINTK
 	bool
 	default y

arch/powerpc/include/asm/checksum.h

@@ -9,9 +9,6 @@
  * 2 of the License, or (at your option) any later version.
  */
 
-#ifdef CONFIG_GENERIC_CSUM
-#include <asm-generic/checksum.h>
-#else
 #include <linux/bitops.h>
 #include <linux/in6.h>
 /*
@@ -217,6 +214,5 @@ __sum16 csum_ipv6_magic(const struct in6_addr *saddr,
 			const struct in6_addr *daddr,
 			__u32 len, __u8 proto, __wsum sum);
 
-#endif
 #endif /* __KERNEL__ */
 #endif