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/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