Remove note saying faulthandler_timeout is not available on Windows

I think it is available in all Python versions we support, but at least
since Python 3.7 the docs[0] say:

    Changed in version 3.7: This function is now always available.

so let's just remove the note.

[0] https://docs.python.org/3/library/faulthandler.html#faulthandler.dump_traceback_later

src/_pytest/faulthandler.py

@@ -13,8 +13,7 @@ fault_handler_stderr_key = StoreKey[TextIO]()
 def pytest_addoption(parser):
     help = (
         "Dump the traceback of all threads if a test takes "
-        "more than TIMEOUT seconds to finish.\n"
-        "Not available on Windows."
+        "more than TIMEOUT seconds to finish."
     )
     parser.addini("faulthandler_timeout", help, default=0.0)