Replace undo with delete-indentation function.

There was such interaction while executing `smart-return' command:
1. new line
2. undo
3. new line
4. undo
5. etc

Now it will be as the following:
1. new line
2. delete-indentation, new line
3. delete-indentation, new line

So there will be no "undo" step while executing `smart-return' command.