libalias: Remove unused function LibAliasCheckNewLink

The functionality to detect a newly created link after processing a
single packet is decoupled from the packet processing. Every new
packet is processed asynchronously and will reset the indicator, hence
the function is unusable. I made a Google search for third party code,
which uses the function, and failed to find one.

That's why the function should be removed: It unusable and unused.
A much simplified API/ABI will remain in anything below 14.

Discussed with: kp
Reviewed by: manpages (bcr)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30275