Drop codecs/xtrn. Removed upstream

Drop codecs/xtrn. Removed upstream

> This coder is a kind of backdoor used by the COM object that allows it to
> pass blobs back and forth using the coder interface. It simply encodes and
> decodes the filename as a comma delimited string and extracts the info it
> needs.

- https://keybase.pub/wikibuda/ImageMagick-7.0.10-9/coders/xtrn.c

Refs:
- https://github.com/ImageMagick/ImageMagick/commit/0d1b03d3176d47e6f2e512bdc7968a9fa153189a
- https://github.com/ImageMagick/ImageMagick/commit/6a863ccebb6d777739fb8d9b5b3b78c868dafb82