BUG: do not generate Thebe files for static sites

[bsipocz]

Description

myst build --html generates a bunch of thebe.js files even if the thebe sections are removed from myst.yml.

(Interestingly they are not generated with when there is executable notebook content and the site is built with myst build --execute --html.)

This is an inconsistent behaviour, and generates unused files that may need to be cleaned up from deployed artifacts (though checking on the cumulative size of the files, it's not significant).