fix: load region from AWS client if the credentials are managed by AWS-SDK

[Amplifiyer]

Description of changes

When using general configLevel for aws credentials, deployment manager was failing because it expected amplify-cli to resolve credentials (and hence region) whereas general config implies that credentials are managed by customers and resolved by aws-sdk automatically without cli doing anything. This change removes the validation and instead defaults to AWS client's region instead of taking it away completely to reduce the scope of this change.

Reference: https://docs.amplify.aws/gen1/react/tools/cli/usage/headless/#--providers-1

See this for a similar example.

Issue #, if available

Description of how you validated changes

Manually tested before and after and confirmed that it fixes the issue.

Checklist

• PR description included
• yarn test passes
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)
• New AWS SDK calls or CloudFormation actions have been added to relevant test and service IAM policies
• Pull request labels are added

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