Remove build files generated with cargo doc command

[adpaco-aws]

Description of changes:

Remove build files to avoid false positives in secret scanning alerts (more details in #2735) in scripts/build-docs.sh.

Resolved issues:

Resolves #2735

Call-outs:

We could instead add an extra step to the kani.yml workflow to do the same. That'd allow us to keep build files when we build the docs locally. But I think that's a rare scenario, and I'd rather keep this in the doc-building script which is already doing similar things.

Testing:

• How is this change tested? Locally.

• Is this a refactor change? No.

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.