don't deduplicate itext ids

Since the uniqueness of itext ids is pretty well assured during normal
usage (with the exception of if you create a question with the ID
foo-bar and then create a select question with the ID foo with an item
value bar), unless you manually edit them, in which case you probably
*don't* want them changed, this isn't really necessary.

(Even in that exceptional case, what would happen is that the itexts
would get linked when you created the second question, and then they
would share the same display text, which would get copied to a new itext
item and renamed when you serialize the form, so ultimately this
deduplication feature only prevents you from unknowingly making changes
to multiple questions, but only after you've saved.)

Also, it makes it impossible to use vellum on a form where you're using
the shared itext strategy to save memory as discussed on field-dev.