selftests: xsk: Remove unused function

Probably it was ported from xdpsock but is not used anywhere.
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210329224316.17793-4-maciej.fijalkowski@intel.com

tools/testing/selftests/bpf/xdpxceiver.c

@@ -153,19 +153,6 @@ static void *memset32_htonl(void *dest, u32 val, u32 size)
 	return dest;
 }
 
-/*
- * This function code has been taken from
- * Linux kernel lib/checksum.c
- */
-static inline unsigned short from32to16(unsigned int x)
-{
-	/* add up 16-bit and 16-bit for 16+c bit */
-	x = (x & 0xffff) + (x >> 16);
-	/* add up carry.. */
-	x = (x & 0xffff) + (x >> 16);
-	return x;
-}
-
 /*
  * Fold a partial checksum
  * This function code has been taken from