libspl/include: remove unused/empty headers

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

include/libzfs.h

@@ -40,10 +40,8 @@
 #include <sys/mnttab.h>
 #include <sys/param.h>
 #include <sys/types.h>
-#include <sys/varargs.h>
 #include <sys/fs/zfs.h>
 #include <sys/avl.h>
-#include <ucred.h>
 #include <libzfs_core.h>
 
 #ifdef	__cplusplus

include/thread_pool.h

@@ -28,7 +28,6 @@
 #define	_THREAD_POOL_H_ extern __attribute__((visibility("default")))
 
 #include <sys/types.h>
-#include <thread.h>
 #include <pthread.h>
 
 #ifdef	__cplusplus