tree: Remove support for allowDuplicates

This feature was only useful in edge cases where the same item could
appear multiple times within a store's hierarchy, and it only ever worked
in a rather limited capacity.  More recently, a change to _StoreMixin was made
to provide more consistent `row` return values, which further impeded this
functionality.

Fully pursuing this functionality would potentially involve complicating core
APIs for the purpose of a rare edge case, so instead we have elected to remove
support for it.  Store items should always contain unique IDs; if you have a
store which conceptually includes the same item at multiple levels of its
hierarchy, you can add logic to your store to generate unique IDs based on
several distinguishing fields.
tree: Update docs for removing allowDuplicates support