gdb: remove internalvar_funcs::destroy

No kind of internal var uses it remove it.  This makes the transition to
using a variant easier, since we don't need to think about where this
should be called (in a destructor or not), if it can throw, etc.

Change-Id: Iebbc867d1ce6716480450d9790410d6684cbe4dd

gdb/break-catch-throw.c

@@ -510,7 +510,6 @@ static const struct internalvar_funcs exception_funcs =
 {
   compute_exception,
   NULL,
-  NULL
 };
 
 

gdb/inferior.c

@@ -982,7 +982,6 @@ static const struct internalvar_funcs inferior_funcs =
 {
   inferior_id_make_value,
   NULL,
-  NULL
 };
 
 

gdb/infrun.c

@@ -9368,7 +9368,6 @@ static const struct internalvar_funcs siginfo_funcs =
 {
   siginfo_make_value,
   NULL,
-  NULL
 };
 
 /* Callback for infrun's target events source.  This is marked when a

gdb/probe.c

@@ -866,7 +866,6 @@ static const struct internalvar_funcs probe_funcs =
 {
   compute_probe_arg,
   compile_probe_arg,
-  NULL
 };
 
 

gdb/thread.c

@@ -2123,7 +2123,6 @@ static const struct internalvar_funcs thread_funcs =
 {
   thread_id_per_inf_num_make_value,
   NULL,
-  NULL
 };
 
 /* Implementation of `gthread' variable.  */
@@ -2132,7 +2131,6 @@ static const struct internalvar_funcs gthread_funcs =
 {
   global_thread_id_make_value,
   NULL,
-  NULL
 };
 
 void _initialize_thread ();

gdb/tracepoint.c

@@ -3985,7 +3985,6 @@ traceframe_available_memory (std::vector<mem_range> *result,
 static const struct internalvar_funcs sdata_funcs =
 {
   sdata_make_value,
-  NULL,
   NULL
 };
 

gdb/value.c

@@ -2449,11 +2449,6 @@ clear_internalvar (struct internalvar *var)
       xfree (var->u.string);
       break;
 
-    case INTERNALVAR_MAKE_VALUE:
-      if (var->u.make_value.functions->destroy != NULL)
-	var->u.make_value.functions->destroy (var->u.make_value.data);
-      break;
-
     default:
       break;
     }

gdb/value.h

@@ -1002,12 +1002,6 @@ struct internalvar_funcs
 			 struct agent_expr *expr,
 			 struct axs_value *value,
 			 void *data);
-
-  /* If non-NULL, this is called to destroy DATA.  The DATA argument
-     passed to this function is the same argument that was passed to
-     `create_internalvar_type_lazy'.  */
-
-  void (*destroy) (void *data);
 };
 
 extern struct internalvar *create_internalvar_type_lazy (const char *name,

gdb/windows-tdep.c

@@ -970,7 +970,6 @@ static const struct internalvar_funcs tlb_funcs =
 {
   tlb_make_value,
   NULL,
-  NULL
 };
 
 /* Layout of an element of a PE's Import Directory Table.  Based on: