MEDIUM: mux-h2: rely on the generic trailers parser

MEDIUM: mux-h2: rely on the generic trailers parser

http_trailers_to_htx() was made from h2_make_htx_trailers(), so it's
expected that we can get rid of the latter in favor of the former to
aim towards a generic parser.

For now we don't exploit the extended return codes, we only check for
HTTP_PRS_SUCCESS to detect parsing failures.