Remove references to TDB2::data_size

This value will is not relevant for TaskChampion.

src/TDB2.cpp

@@ -1285,12 +1285,6 @@ int TDB2::num_reverts_possible ()
   return std::count(lines.begin(), lines.end(), "---");
 }
 
-////////////////////////////////////////////////////////////////////////////////
-size_t TDB2::data_size ()
-{
-  return pending._file.size () + completed._file.size () + undo._file.size () + backlog._file.size ();
-}
-
 ////////////////////////////////////////////////////////////////////////////////
 void TDB2::dump ()
 {