timers: remove dead code and simplify args check

The `setUnrefTimeout` function is never called with more arguments
than two. So quite some code was dead and never used. This removes
that code and simplifies the args check not to coerce objects to
booleans.

PR-URL: https://github.com/nodejs/node/pull/26555


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>