mips: remove dead comment and definitions.

Reviewed by:	brooks, jhb
Sponsored by:	The FreeBSD Foundation
MFC after:	3 days
Differential revision:	https://reviews.freebsd.org/D19584

sys/mips/mips/vm_machdep.c

@@ -453,14 +453,6 @@ cpu_set_upcall(struct thread *td, void (*entry)(void *), void *arg,
 	 */
 }
 
-/*
- * Implement the pre-zeroed page mechanism.
- * This routine is called from the idle loop.
- */
-
-#define	ZIDLE_LO(v)	((v) * 2 / 3)
-#define	ZIDLE_HI(v)	((v) * 4 / 5)
-
 /*
  * Software interrupt handler for queued VM system processing.
  */