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No longer used anywhere. Also removing IronValidatableBehavior which
was only used by iron-input, and is now also unused.

Fixed: 899982
Change-Id: I12cedab34ee8ab62e0b40ef4e69d67970d123f2c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2238997Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Auto-Submit: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#777255}