zstream: remove duplicate highbit64 definition

When building a static build (--disable-shared), zstream fails to link
because of the duplicate highbit64() in libzpool/kernel.c. Since they're
identical, and the libzpool one is visible to zstream, we remove
zstream's copy and just use the common one.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes openzfs#16426

cmd/zstream/zstream_redup.c

@@ -56,17 +56,6 @@ typedef struct redup_table {
 	int		numhashbits;
 } redup_table_t;
 
-#ifndef _WIN32
-int
-highbit64(uint64_t i)
-{
-	if (i == 0)
-		return (0);
-
-	return (NBBY * sizeof (uint64_t) - __builtin_clzll(i));
-}
-#endif
-
 void *
 safe_calloc(size_t n)
 {