Improve local dev experience with `aws:sdk:assemble` build target (#2859)

In local dev, if you have a terminal/shell open to
`aws/sdk/build/aws-sdk/...` and run the `aws:sdk:assemble` target, that
shell's current working directory will break and you'll have to change
directory to somewhere safe and change back in order to do anything
again (at least on MacOS).

This PR makes the `deleteSdk` target that `aws:sdk:assemble` depends on
only delete files instead of directories so that the current working
directory doesn't get invalidated.

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