Drop handling for PHP_CURL_HTTP_REMOVE_SERVER

Historically it was necessary to use a separate server to test
curl against. For many years now we use the builtin server instead.
I don't believe there's much point in retaining this functionality.

ext/curl/tests/server.inc

@@ -1,10 +1,6 @@
 <?php declare(strict_types=1);
 
 function curl_cli_server_start() {
-    if(getenv('PHP_CURL_HTTP_REMOTE_SERVER')) {
-        return getenv('PHP_CURL_HTTP_REMOTE_SERVER');
-    }
-
     $php_executable = getenv('TEST_PHP_EXECUTABLE');
     $doc_root = __DIR__;
     $router = "responder/get.inc";

ext/curl/tests/skipif.inc

@@ -1,7 +1,2 @@
 <?php
-    if (!extension_loaded("curl")) exit("skip curl extension not loaded");
-    if(false === getenv('PHP_CURL_HTTP_REMOTE_SERVER')) {
-        if (php_sapi_name() != "cli") {
-                die("skip PHP_CURL_HTTP_REMOTE_SERVER env variable is not defined");
-        }
-    }
+if (!extension_loaded("curl")) exit("skip curl extension not loaded");