Rename getCancelRoute to getCancelUrl

In keeping with

https://www.drupal.org/node/2305023

None of the cancellation forms will work until this is fix.

It is also critical because stops simplereservation from being installed. I observe the following:-

1) drush install leads to errors.

OR

2) web-install leads to WSOD.

NB. I have change one of the cancel routes ...
to make more sense the delete button on
admin/config/services/simplereservation/nodetype when cancel is presses it returns you to the correct page.

I have improved the comments and the indentation of SimpleReservationReservationDeleteForm::submitForm()

NB There are other critical issues with this module which prevent testing of the issue but they are being resolved elsewhere.