sysutils/terraform-docs: drop $FreeBSD$ again after bbd7f0fac793

$FreeBSD$ was originally removed in 305f148f482d (2021-04-06) which
confused portlint until f36c30ba1869 (2021-04-19), so a workaround
accidentally crept in into the patch.

$ portlint -C
FATAL: Makefile: [1]: $FreeBSD$ is deprecated in Git.

PR: 255459