Remove tre from the vendor tree

This was brought in and used in a user branch for a GSoC 2011 project;
ultimately, we've not moved in this direction and I suspect that we'll
likely instead move towards onigmo or something similar for a regex(3)
replacement that can also double as a speedy libregex with support for GNU
extensions.

The version of tre that was imported separately with bsdgrep was removed in
r333236 due to it ultimately being fairly buggy and no obvious gain on
modern-ish machines. Further, bsdgrep will instead want libregex for its GNU
extensions ocne that project has finished.