Template: remove undisplayed data tracking
Template: remove undisplayed data tracking

This sanity check was added in 533881bf88 to help catch places where
_processing_ pages were assigning to the Template, which would never
actually get displayed.

We no longer need this for several reasons:

1. All known violations of this check have been fixed in the interim.

2. We are soon moving to an engine design that makes the distinction
   between display and processing pages much more explicit, so there
   will then be less ambiguity about when the Template can and cannot
   be assigned to.

3. The case of throwing an error in the middle of executing a display
   page was not handled properly. In such a case, we don't care what
   variables were assigned to the Template previously, because we now
   want to display an error page. This caused us to have to shuffle
   around Template assignments into less intuitive locations (so that
   all error checking was performed first, e.g. fd1c54c77c52).

smrealms/smr,smrealms/smr,smrealms/smr