url: Refactor detect_proxy()

If defined CURL_DISABLE_HTTP, detect_proxy() returned NULL. If not
defined CURL_DISABLE_HTTP, detect_proxy() checked noproxy list.

Thus refactor to set proxy to NULL instead of calling detect_proxy() if
define CURL_DISABLE_HTTP, and refactor to call detect_proxy() if not
define CURL_DISABLE_HTTP and the host is not in the noproxy list.