Correct once handling for "nested" events

If you have two once listeners and the first one dispatches a second event that invokes both listeners, the second listener would be invoked twice as it had already been cloned before it got removed. As such setting its removed field is important after all.

Fixes #1323.

dom.bs

@@ -1608,10 +1608,8 @@ and an optional <var>legacyOutputDidListenersThrowFlag</var>, run these steps:
    <a for="event listener">capture</a> is true, then <a for=iteration>continue</a>.
 
    <li><p>If <var>listener</var>'s <a for="event listener">once</a> is true, then
-   <a for=list>remove</a> <var>listener</var> from <var>event</var>'s {{Event/currentTarget}}
-   attribute value's <a for=EventTarget>event listener list</a>.
-   <!-- Do this before invocation to avoid reentrancy issues. No need to set removed to true since
-        each listener in listeners is run once anyway. -->
+   <a>remove an event listener</a> given <var>event</var>'s {{Event/currentTarget}} attribute value
+   and <var>listener</var>.
 
    <li><p>Let <var>global</var> be <var>listener</var> <a for="event listener">callback</a>'s
    <a>associated realm</a>'s <a for=realm>global object</a>.
@@ -10387,6 +10385,7 @@ Ahmid <i>snuggs</i>,
 Alex Komoroske,
 Alex Russell,
 Alexey Shvayka,
+Andreas Kling<!-- awesomekling; GitHub -->,
 Andreu Botella,
 Anthony Ramine,
 Arkadiusz Michalski,