libspl/include: remove unused/empty headers

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13447

lib/libzfs/libzfs_dataset.c

@@ -49,7 +49,6 @@
 #include <sys/mount.h>
 #include <pwd.h>
 #include <grp.h>
-#include <ucred.h>
 #ifdef HAVE_IDMAP
 #include <idmap.h>
 #include <aclutils.h>

lib/libzfs/libzfs_diff.c

@@ -41,7 +41,6 @@
 #include <unistd.h>
 #include <stdio.h>
 #include <stdlib.h>
-#include <stropts.h>
 #include <pthread.h>
 #include <sys/zfs_ioctl.h>
 #include <libzfs.h>