doc: remove outdated timeout.unref content

resolve: nodejs#42239

PR-URL: nodejs#42241
Fixes: nodejs#42239
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Mestery <mestery@protonmail.com>

Author: meixg

doc/api/timers.md

@@ -140,10 +140,6 @@ to remain active. If there is no other activity keeping the event loop running,
 the process may exit before the `Timeout` object's callback is invoked. Calling
 `timeout.unref()` multiple times will have no effect.
 
-Calling `timeout.unref()` creates an internal timer that will wake the Node.js
-event loop. Creating too many of these can adversely impact performance
-of the Node.js application.
-
 ### `timeout[Symbol.toPrimitive]()`
 
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