n-api: remove idle_running from TsFn

The idle_running member variable in TsFn is always false and can
therefore be removed.

PR-URL: https://github.com/nodejs/node/pull/22520


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: Gus Caplan <me@gus.host>