loader/lua: Remove workaround for command_error

loader.command_error was available prior to stable/12 branching. No need
to check if it is available or not.

Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D44144

(cherry picked from commit 552f3072af54820cf1805f712e2567bc1b7f046d)