MINOR: htx: Make internal.strm.is_htx an internal sample fetch

MINOR: htx: Make internal.strm.is_htx an internal sample fetch

It is not really a context-less sample fetch, but it is internal. And it
only fails if no stream is attached to the sample. This way, it is still
possible to use it on an HTTP proxy (L6 sample fetches are ignored now for
HTTP proxies).

If the commit "BUG/MINOR: payload/htx: Ingore L6 sample fetches for HTX
streams/checks" is backported, it may be a good idea to backport this one
too. But only as far as 2.2.