Don't force symbol to have a library (KICAD-74K).

(cherry picked from commit 233da7934d)

eeschema/tools/symbol_editor_control.cpp

@@ -848,8 +848,6 @@ int SYMBOL_EDITOR_CONTROL::AddSymbolToSchematic( const TOOL_EVENT& aEvent )
             return 0;
         }
 
-        wxCHECK( libSymbol->GetLibId().IsValid(), 0 );
-
         SCH_SYMBOL* symbol = new SCH_SYMBOL( *libSymbol, libId, &schframe->GetCurrentSheet(),
                                              unit, bodyStyle );