urldata: remove the aptr struct

Move the two remaining fields into the state struct instead, within the
HTTP ifdef.

Also: make the AWS sigv4 code always rely on the http_host value. It
was previously conditionally trying to also work if it was not set, but
that logic was not creating an identical copy so it would fail.

Closes #22620

lib/http_proxy.c

@@ -132,7 +132,7 @@ static CURLcode dynhds_add_custom(struct Curl_easy *data,
       /* trim surrounding whitespace so a padded field name (e.g.
          `Authorization :`) cannot slip past the Authorization/Cookie check */
       curlx_str_trimblanks(&name);
-      if(data->state.aptr.host &&
+      if(data->state.http_host &&
          /* a Host: header was sent already, do not pass on any custom Host:
             header as that will produce *two* in the same request! */
          curlx_str_casecompare(&name, "Host"))