Remove the now unused IterableBitSet class.

Bug: angleproject:3877
Change-Id: I161e5d062c8183e30a7eb9040f3018116fe6e69e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2683494
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>