test_concurrent_futures: Fix unneeded/confusing format call (GH-93119)

Added in 339fd46cb7 - but as noted in a comment, the test only tests ThreadPoolExecutor.
(cherry picked from commit 3f2dd0a7c0)

Co-authored-by: Florian Bruhin <me@the-compiler.org>

Lib/test/test_concurrent_futures.py

@@ -485,7 +485,7 @@ def test_cancel_futures_wait_false(self):
                 t = ThreadPoolExecutor()
                 t.submit(sleep_and_print, .1, "apple")
                 t.shutdown(wait=False, cancel_futures=True)
-            """.format(executor_type=self.executor_type.__name__))
+            """)
         # Errors in atexit hooks don't change the process exit code, check
         # stderr manually.
         self.assertFalse(err)