Remove old load_lang() file fallback due to security issues.

The interface for this is already gone too. Has been since
4.5.0.

It does not sanitize the filename, but is used to process
user defined values. It neither implements the whole
parser.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5721 2fea6a4d-a838-0410-917b-93a53c48e9d2
Brand message as an error.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5722 2fea6a4d-a838-0410-917b-93a53c48e9d2