Remove stale M_ZOMBIE malloc type.

This type is unused since embedding p_ru into struct proc.

MFC after:	1 week

sys/kern/kern_exit.c

@@ -94,9 +94,6 @@ SDT_PROVIDER_DECLARE(proc);
 SDT_PROBE_DEFINE(proc, kernel, , exit, exit);
 SDT_PROBE_ARGTYPE(proc, kernel, , exit, 0, "int");
 
-/* Required to be non-static for SysVR4 emulator */
-MALLOC_DEFINE(M_ZOMBIE, "zombie", "zombie proc status");
-
 /* Hook for NFS teardown procedure. */
 void (*nlminfo_release_p)(struct proc *p);
 

sys/sys/proc.h

@@ -664,7 +664,6 @@ MALLOC_DECLARE(M_PARGS);
 MALLOC_DECLARE(M_PGRP);
 MALLOC_DECLARE(M_SESSION);
 MALLOC_DECLARE(M_SUBPROC);
-MALLOC_DECLARE(M_ZOMBIE);
 #endif
 
 #define	FOREACH_PROC_IN_SYSTEM(p)					\