Remove LUA_LIBDIR from PLIST_SUB.

[mat]

Summary

It contains only lib and gets replaced in so many wrong places it is
a bad idea to have it.

[tobik]

There is a workaround for it in games/freeciv to avoid cluttering its pkg-plist that should be removed too now.

[mat]

In D29138#652567, @tobik wrote:

There is a workaround for it in games/freeciv to avoid cluttering its pkg-plist that should be removed too now.

Ah, thank you, I had missed it.