timers: remove domain enter and exit

With domains implemented over AsyncHooks, it's no longer
necessary to explicitly enter and exit the domain.

PR-URL: https://github.com/nodejs/node/pull/17880


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>
Reviewed-By: Andreas Madsen <amwebdk@gmail.com>