loader/lua: Remove compat shim for loader.lua_path

loader.lua_path was committed before stable/13 was branched, and merged
in to for 12.2. Remove workaround for it not being present.

Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D44143

(cherry picked from commit ab97d42addae97a389c6f22d6bba62ed954bb8e7)