process: remove undocumented `now` argument from emitWarning()

process.emitWarning() "now" option is undocumented and a Boolean trap.
Remove it before people start adopting it.

We only need it in one place internally. Replace it with an
internal-only emitWarningSync() function.

PR-URL: https://github.com/nodejs/node/pull/31643


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Myles Borins <myles.borins@gmail.com>