Backport of agent: remove unused log filter and unrequired library. into release/1.9.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #24873 to be assessed for backporting due to the inclusion of the label backport/1.9.x.

The below text is copied from the body of the original PR.

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The Nomad agent used a log filter to ensure logs were written at the expected level. Since the use of hclog this is not required, as hclog acts as the gate keeper and filter for logging. All log writers accept messages from hclog which has already done the filtering.

Double filtering has been removed from syslog in previous PR's. The full removal in this PR, and from the log file implementation will give this log feature a tiny performance improvement.

The change allows us to remove an external library; albeit a small one.

Testing & Reproduction steps

Can be tested via a dev agent. It touches the agent logging as a whole, so worth trying a few different combinations of level and logging type, especially log-file.

Contributor Checklist

• Changelog Entry If this PR changes user-facing behavior, please generate and add a
  changelog entry using the make cl command.
• Testing Please add tests to cover any new functionality or to demonstrate bug fixes and
  ensure regressions will be caught.
• Documentation If the change impacts user-facing functionality such as the CLI, API, UI,
  and job configuration, please update the Nomad website documentation to reflect this. Refer to
  the website README for docs guidelines. Please also consider whether the
  change requires notes within the upgrade guide.

Reviewer Checklist

• Backport Labels Please add the correct backport labels as described by the internal
  backporting document.
• Commit Type Ensure the correct merge method is selected which should be "squash and merge"
  in the majority of situations. The main exceptions are long-lived feature branches or merges where
  history should be preserved.
• Enterprise PRs If this is an enterprise only PR, please add any required changelog entry
  within the public repository.

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Overview of commits

• 753f752