Legacy checks removal

[denis256]

Description

Noticed a couple of invalid checks:

aws/resources/kms_customer_key.go:64:10: impossible condition: nil != nil
aws/resources/lambda_layer.go:134:10: impossible condition: nil != nil
aws/resources/security_hub.go:108:9: tautological condition: nil == nil

TODOs

Read the Gruntwork contribution guidelines.

• [x] Update the docs.
• [x] Run the relevant tests successfully, including pre-commit checks.
• [x] Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• [x] Include release notes. If this PR is backward incompatible, include a migration guide.
• [x] Attention Grunts - if this PR adds support for a new resource, ensure the nuke_sandbox and nuke_phxdevops jobs in .circleci/config.yml have been updated with appropriate exclusions (either directly in the job or via the .circleci/nuke_config.yml file) to prevent nuking IAM roles, groups, resources, etc that are important for the test accounts.

Release Notes (draft)

Added / Removed / Updated [X].

Removed legacy code

Migration Guide