Spreadsheet: Don't remove on_change tab function after loading a file

Forgot to remove that in c0c9825f67, as
this function was no longer declared and used... Until the previous
commit.

This meant that pressing the F2 key after opening a file no longer
matched the current tab.

Userland/Applications/Spreadsheet/SpreadsheetWidget.cpp

@@ -425,7 +425,6 @@ void SpreadsheetWidget::load_file(Core::File& file)
         return;
     }
 
-    m_tab_widget->on_change = nullptr;
     m_cell_value_editor->on_change = nullptr;
     m_current_cell_label->set_text("");
     m_should_change_selected_cells = false;