DELETE VARIABLES: Fix crash with FILTER.

DELETE VARIABLES: Fix crash with FILTER.

FILTER has the surprising property that it introduces a temporary
transformation, even if proc_make_temporary_transformations_permanent()
was previously called.  This causes a crash if the procedure implementation
really relies on there being no temporary transformations.

This commit fixes the problem in the procedures I was able to identify as
having it, adds a comment to proc_make_temporary_transformations_permanent()
to make it a little less likely to happen again, and adds a test
specifically to check for regression on DELETE VARIABLES.

Reported by John Darrington.
Bug #38843.