refactor: remove unused methods {CDBIterator,CCoinsViewDBCursor}::GetValueSize()

These methods haven't been used since the chainstate db cache has been
switched from per-tx to per-txout model years ago (PR #10195, commit
d342424301).

src/dbwrapper.h

@@ -166,11 +166,6 @@ public:
         }
         return true;
     }
-
-    unsigned int GetValueSize() {
-        return piter->value().size();
-    }
-
 };
 
 class CDBWrapper