ci(doc-linter): delete lint_mac dead code

`lint_mac()` was called and its return value immediately overwritten
with `None`:

    err_mac = lint_mac(cnt, line.strip())
    # disable mac detection for the moment, too many false positives
    err_mac = None

The comment is correct — MAC linting produced too many false
positives — but the cleanup never landed. The dead call ran on every
line for every linted file, and the function/MAC-regex/MAC-error-text
all sat in the source as a misleading hint that MAC linting was a
live feature.

Delete:
- the `MAC` regex constant
- the `lint_mac()` function
- the `err_mac = lint_mac(...)`, `err_mac = None`, and the `err_mac`
  entry in the tuple iterated by the error-collection loop in
  `handle_file_action()`

Tracked as item 9 of the rolling-side cleanup backlog flagged across
the PR #2014 / #2019 / #2020 reviews. When MAC linting is genuinely
wanted again, recover the regex/function from git history and wire it
in cleanly.

🤖 Generated by [robots](https://vyos.io)

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