libctf: remove unused variable

This fixes the build with gcc 16 after the changes
to -Wunused*[0].

[0] https://gcc.gnu.org/gcc-16/porting_to.html#changes-to-wunused

Reviewed by:	markj
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D59537

cddl/contrib/opensolaris/common/ctf/ctf_hash.c

@@ -77,9 +77,8 @@ ctf_hash_compute(const char *key, size_t len)
 {
 	ulong_t g, h = 0;
 	const char *p, *q = key + len;
-	size_t n = 0;
 
-	for (p = key; p < q; p++, n++) {
+	for (p = key; p < q; p++) {
 		h = (h << 4) + *p;
 
 		if ((g = (h & 0xf0000000)) != 0) {