security/maltrail: don't override the Check Hostheaders setting

[Lux-WorldPC]

Important notices

Before you submit a pull request, we ask you kindly to acknowledge the following:

• I have read the contributing guidelines at https://github.com/opnsense/plugins/blob/master/CONTRIBUTING.md
• I opened an issue first for non-trivial changes and linked it below.
• AI tools were used to create at least part of the code submitted herewith.

If AI was used, please disclose:

• Model used: Claude Opus 5 (via Claude Code)
• Extent of AI involvement: used to locate the offending line, to check maltrail upstream
  (core/settings.py) for the duplicate-key behaviour, and to trace the template history.
  The fix is the one-line deletion proposed by the reporter in os-maltrail: CHECK_HOST_DOMAINS appears twice #5489. I reviewed and verified
  each of those points before opening this PR.

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Describe the problem

CHECK_HOST_DOMAINS is emitted twice in the generated maltrail.conf: once from the
checkhostheader conditional, and again a few lines below as a hardcoded false.

maltrail's read_config() walks the file top to bottom and ends each iteration with a plain
config[name] = value, with no guard, so the last occurrence wins. The trailing hardcoded line
therefore always overrides the GUI setting and pins CHECK_HOST_DOMAINS to false.

The hardcoded line dates back to the initial template. The conditional block was later inserted
directly above it in cdf3620 ("security/maltrail: add hostheader checking", #3144) without removing
it, so Services -> Maltrail -> General -> Check Hostheaders has had no effect since the option was
introduced.

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Describe the proposed solution

Drop the leftover hardcoded CHECK_HOST_DOMAINS false so the emitted value is the one selected in
the GUI. This is the fix suggested by @thwien in the issue.

Users who had the toggle off are unaffected (the conditional still emits false); users who had it
on now get the true they asked for.

I left PLUGIN_REVISION alone, assuming the bump is done on your side as usual - happy to add it if
you prefer.

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Related issue

Closes #5489