Remove support for multipart/x-mixed-replace

LayoutTests/http/tests/multipart/multipart-html.php

-<?php
-header('Content-type: multipart/x-mixed-replace; boundary=boundary');
-header('Connection: keep-alive');
-echo "--boundary\r\n";
-echo "Content-Type: text/html\r\n\r\n";
-echo str_pad('', 5000);
-?>
-
-<script>
-if (window.testRunner)
-    testRunner.dumpAsText();
-</script>
-
-<?php
-for ($i = 0; $i <= 10; $i++) {
-    echo "--boundary\r\n";
-    echo "Content-Type: text/html\r\n\r\n";
-    echo "This text should only appear once ";
-    echo $i;
-    echo str_pad('', 5000);
-    echo "\r\n\r\n";
-    flush();
-    usleep(100000);
-    $i++;
-}
-?>