redundant includes

Remove several duplicate includes I discovered while reconciling the
vms Makefile.  All of these are already being brought in via hack.h so don't
need to be explicitly included after it.

src/mkobj.c

@@ -3,7 +3,6 @@
 /* NetHack may be freely redistributed.  See license for details. */
 
 #include "hack.h"
-#include "prop.h"
 
 STATIC_DCL void FDECL(mkbox_cnts,(struct obj *));
 STATIC_DCL void FDECL(obj_timer_checks,(struct obj *, XCHAR_P, XCHAR_P, int));