Remove failing test erroneously added in backport

* test/src/thread-tests.el (threads-test-bug33073): Remove
test which cannot work on the emacs-26 branch.  Do not merge
to master.  Reported by Juanma Barranquero <lekktu@gmail.com>.
1 job for emacs-26 in 28 minutes and 30 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #2166
test

00:28:30