events: return values directly in listeners

Each conditional branch in EventEmitter.prototype.listeners assigns
its return value to a variable ret which is returned at the end.
Instead just return from within each branch. This is both clearer
and more performant.

events/ee-listeners.js n=5000000      3.65 %        *** 3.359171e-10

PR-URL: https://github.com/nodejs/node/pull/16212


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>