Remove support for multipart/x-mixed-replace

LayoutTests/http/tests/multipart/resources/multipart-wait-before-boundary.php

-<?php
-    if (isset($_GET['done'])) {
-      header("Content-Type: text/html");
-      echo("<script>parent.success()</script>");
-      exit(0);
-    }
-
-    $boundary = "cutHere";
-
-    function sendHeader()
-    {
-        global $boundary;
-
-        echo("--$boundary\r\n");
-        echo("Content-Type: text/html\r\n\r\n");
-        flush();
-    }
-
-    header("Content-Type: multipart/x-mixed-replace; boundary=$boundary");
-
-    // generate some padding to work around CFNetwork handling of multipart data
-    $padding = "aa";
-    for ($i = 0; $i < 10; $i++) {
-      $padding .= $padding;
-    }
-
-    sendHeader();
-    ob_end_flush();
-    echo("test html\n");
-    echo("<!-- $padding -->");
-    flush();
-    sendHeader();
-    echo("second html");
-    echo("<script>parent.childLoaded()</script>");
-    echo("<!-- $padding -->");
-    flush();
-    sendHeader();
-    echo("third html");
-    echo("<!-- $padding -->");
-    flush();
-    usleep(20 * 1000000);
-?>