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/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
 };