task-thai-desktop: Stop recommending xiterm+thai

This was originally added in 2003 with e6d3637a. My guess is that anything related to Thai was included then. Since then, Unicode has become even more of a standard. Therefore, this app does not appear to be important enough to be installed by default for everyone using Debian in Thai.

Debian Live currently installs all the language tasks to try to make things more useful for people with various preferred languages.

It is reported that this app doesn't work in some environments such as LXDE (maybe some additional configuration is needed there).

I asked for feedback from some Debian Developers who I think use Thai in https://bugs.debian.org/1110661, in part because I'm wondering about fixing this for all desktop flavors of Debian Live for future Debian 13 point releases. I don't think we need to wait for a response before making this change for Debian Unstable and Testing though. It's easy to revert if necessary and I think there are reasonable arguments to be hesitant about including GUI apps in the language tasks.