http: move OutboundMessage.prototype.flush to EOL

API was deprecated long ago. Move to end of life and remove.

PR-URL: #31164
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

doc/api/deprecations.md

@@ -44,6 +44,9 @@ However, the deprecation identifier will not be modified.
 ### DEP0001: `http.OutgoingMessage.prototype.flush`
 <!-- YAML
 changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/31164
+    description: End-of-Life.
   - version:
     - v4.8.6
     - v6.12.0
@@ -54,9 +57,9 @@ changes:
     description: Runtime deprecation.
 -->
 
-Type: Runtime
+Type: End-of-Life
 
-The `OutgoingMessage.prototype.flush()` method is deprecated. Use
+`OutgoingMessage.prototype.flush()` has been removed. Use
 `OutgoingMessage.prototype.flushHeaders()` instead.
 
 <a id="DEP0002"></a>

lib/_http_outgoing.js

@@ -900,10 +900,6 @@ OutgoingMessage.prototype.flushHeaders = function flushHeaders() {
   this._send('');
 };
 
-OutgoingMessage.prototype.flush = internalUtil.deprecate(function() {
-  this.flushHeaders();
-}, 'OutgoingMessage.flush is deprecated. Use flushHeaders instead.', 'DEP0001');
-
 OutgoingMessage.prototype.pipe = function pipe() {
   // OutgoingMessage should be write-only. Piping from it is disabled.
   this.emit('error', new ERR_STREAM_CANNOT_PIPE());