[do-not-merge] Remove leave questions

Following yesterday's decision by John/Nick/Chris/Roo, we are deleting the leave questions from the SPL questioning process. 

This PR is to be review commit by commit. 

1) Removes all leave questions, some content changes with "todo" made to snippets to silence errors caused by missing date_leave variables
2) Go through birth factcheck tables and amend flows to correct any legal requirement errors I could spot 

Still to do:

3) Adoption factcheck
4) New content that reflects lack of date_leave information for snippets