test: remove flaky designation for test_threadsafe_function

The test_threadsafe_function doesn't seem to be flaky anymore on
Windows. Optimistically removing the flaky designation in the relevant
status file.

Refs: https://github.com/nodejs/node/issues/23621#issuecomment-468938980

PR-URL: https://github.com/nodejs/node/pull/26403


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>