Delete obsolete stubs for Vec and related options

[zhassan-aws]

Description of changes:

The Vec stubs have been obsolete for 2+ years, are not getting tested in our CI, and the related options (--use-abs and --abs-type) don't work. The stubs code has bugs (e.g. the one pointed out in #2766). Since we're currently taking a different approach for stubbing (stubbing functions and methods as opposed to types), this PR removes all the Vec stubs and related functionality.

Resolved issues:

Resolves #697
Resolves #2766

Related RFC:

Optional #ISSUE-NUMBER.

Call-outs:

Testing:

• How is this change tested?

• 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.

[celinval]

Thank you! Can you please document this as a major change to the release notes? I don't think we need to add them to breaking changes, since this was an unstable option.

[zhassan-aws]

Can you please document this as a major change to the release notes?

Done