Remove incorrect defaults for iot (#3256)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->
The IoT Smithy model we have is incorrect, resulting in some types being
generated as non-optional when they should be optional.

## Description
<!--- Describe your changes in detail -->
Removes the defaults of a few shapes for iot, which are meant to be
nullable. Can remove this when the model is fixed upstream.

Also changed some of the other shapes in RemoveDefaultsDecorator to use
the synthetic shape id, but this didn't impact generated code - just for
consistency.

## Testing
<!--- Please describe in detail how you tested your changes -->
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->
- [x] Generated IoT SDK and verified diff is expected

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

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