remove last remaining reference to _REENTRANT macro and fix DLZ example

It was used only as guard against unused variable declaration, but the
surrounding code depends on strtok_r being defined unconditionally, so
there is no point in guarding a variable.
Glibc documentation suggests it is obsolete anyway and e.g. Meson build
system decided to ignore it. It seems to be required only by old
Solaris compiler and OpenIndiana uses gcc.

contrib/dlz/example/dlz_example.c

@@ -692,9 +692,7 @@ modrdataset(struct dlz_example_data *state, const char *name,
 	char *full_name, *dclass, *type, *data, *ttlstr, *buf;
 	char absolute[1024];
 	isc_result_t result;
-#if defined(_REENTRANT)
 	char *saveptr = NULL;
-#endif /* defined(_REENTRANT) */
 
 	buf = strdup(rdatastr);
 	if (buf == NULL) {