Fix deleting non-existing stack dependency references

[marcinwyszynski]

Description of the change

If the stack or the dependency is removed manually, it will not be possible to delete stack dependency reference. Throughout the rest of the provider we use a consistent pattern of just removing these resources from the state, and not treating it as an error. We will emit a warning here in case anyone misses the old error messages, but otherwise let's have this resource behave like all other resources.

Type of change

• Bug fix (non-breaking change that fixes an issue)

Checklists

Development

• Lint rules pass locally
• The code changed/added as part of this pull request has been covered with tests
• All tests related to the changed code pass in development
• Examples for new resources and data sources have been added
• Default values have been documented in the description (e.g., "Dummy: (Boolean) Blah blah. Defaults to false.)
• If the action fails that checks the documentation: Run go generate to make sure the docs are up to date

Code review

• This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• Pull Request is no longer marked as "draft"
• Reviewers have been assigned
• Changes have been reviewed by at least one other engineer