vm no longer measures bogoclock or times, relies on os for that

* libguile/vm.h (struct scm_vm): Remove "time" and "clock" members. The
  time was bogusly measured, and the "clock" measured instructions
  retired, which is not a very useful measurement, and it was causing
  lots of memory accesses. Not that I have done a proper profile,
  though...
  (scm_vm_stats): Remove this procedure, which provided access to "time"
  and "clock".

* libguile/vm.c:
* libguile/vm-engine.h:
* libguile/vm-engine.c:
* libguile/vm-i-system.c: Adapt to scm_vm changes and scm_vm_stats
  removal.

* module/system/repl/command.scm:
* module/system/vm/vm.scm: Adapt to vm-stats removal by removing
  vm-stats from <repl>.

libguile/vm-engine.h

@@ -339,13 +339,6 @@ do {						\
 #define FETCH()		(*ip++)
 #define FETCH_LENGTH(len) do { len=*ip++; len<<=8; len+=*ip++; len<<=8; len+=*ip++; } while (0)
 
-#undef CLOCK
-#if VM_USE_CLOCK
-#define CLOCK(n)	vp->clock += n
-#else
-#define CLOCK(n)
-#endif
-
 #undef NEXT_JUMP
 #ifdef HAVE_LABELS_AS_VALUES
 #define NEXT_JUMP()		goto *jump_table[FETCH () & SCM_VM_INSTRUCTION_MASK]
@@ -355,7 +348,6 @@ do {						\
 
 #define NEXT					\
 {						\
-  CLOCK (1);					\
   NEXT_HOOK ();					\
   CHECK_STACK_LEAK ();                          \
   NEXT_JUMP ();					\