Remove legacy container metrics collection code

[vboulineau]

What does this PR do?

Clean up old metrics collection code: most of the code was unused since migration to v2, two other components were still calling the old containers package:

• Hostname resolution for the UTS mode check: the check has been moved to use UTS NS inode identifier, which will be more reliable and work everywhere (not only Docker)
• ECS v1 was using GetDefaultGateway in an attempt to find ECS Agent IP, this code has been migrated to the util/system package.

Motivation

Cleanup

Additional Notes

Possible Drawbacks / Trade-offs

Describe how to test/QA your changes

There's nothing specific to QA, but we want to make sure key features that were historically using containers package still work, namely:

• ECS Linux & Windows
• Docker
• Hostname resolution on Docker without Docker socket mounted and --uts host
• Hostname resolution on Kubernetes without Kubelet access and without CRI socket mounted

Reviewer's Checklist

• [x] If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
• [x] Use the major_change label if your change either has a major impact on the code base, is impacting multiple teams or is changing important well-established internals of the Agent. This label will be use during QA to make sure each team pay extra attention to the changed behavior. For any customer facing change use a releasenote.
• [x] A release note has been added or the changelog/no-changelog label has been applied.
• [x] Changed code has automated tests for its functionality.
• [x] Adequate QA/testing plan information is provided if the qa/skip-qa label is not applied.
• [x] At least one team/.. label has been applied, indicating the team(s) that should QA this change.
• [x] If applicable, docs team has been notified or an issue has been opened on the documentation repo.
• [x] If applicable, the need-change/operator and need-change/helm labels have been applied.
• [x] If applicable, the k8s/<min-version> label, indicating the lowest Kubernetes version compatible with this feature.
• [x] If applicable, the config template has been updated.