raw plugin: remove unused function declaration

[SiskaPavel]

Summary

Remove unused function declaration.
Function had unnecessary dependency on pcap/pcap.h

✅ Checks

• Relevant documentation was updated (if needed)
• CI pipeline passes (build, tests, lint, static analysis)
• New code follows clang-tidy rules
• The change is clearly related to a specific issue / requirement
• Documentation (e.g. Doxygen) is updated if needed
• The PR is rebased on the latest master
• No unrelated changes are included

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