Remove redundant url_buffer field

Use url.scheme.ptr directly for freeing the allocated URL string,
since the scheme is always at the start of a URL. This saves 8 bytes
per connection and simplifies the code.

chttp.h

@@ -895,9 +895,7 @@ typedef struct {
     bool trace_bytes;
 
     // Parsed URL for connection establishment
-    // The url_buffer contains the allocated copy of the URL string,
-    // and url.* pointers reference into this buffer
-    char *url_buffer;
+    // url.scheme.ptr points to the allocated copy of the URL string
     HTTP_URL url;
 
     // Data received from the server