MINOR: http: Don't store raw HTTP errors in chunks anymore

MINOR: http: Don't store raw HTTP errors in chunks anymore

Default HTTP error messages are stored in an array of chunks. And since the HTX
was added, these messages are also converted in HTX and stored in another
array. But now, the first array is not used anymore because the legacy HTTP mode
was removed.

So now, only the array with the HTX messages are kept. The other one was
removed.