REGTEST: remove unexpected "nbthread" statement from Lua test cases

Lua test files 2 and 3 fail when threads are disabled because of a
"nbthread" statement that seems to be a leftover from an ancient
configuration. One of them even mentions a commit message showing
a reproducer not involving threads. Let's clean this up so that
running the tests without threads also works.

This should be backported to 1.9 as the problem also exists there.
2 files changed