CLEANUP: mux-h1: remove the unneeded test on conn->owner in h1s_finish_detach()

CLEANUP: mux-h1: remove the unneeded test on conn->owner in h1s_finish_detach()

There was a test below the "release" label on conn->owner to decide
whether to kill the connection or not. But this test is not needed,
because:
  - for frontends, it's always set so the test never matches
  - for backends, it was NULL on the second stream once a request
    was being reused from an idle pool, so it couldn't be used to
    discriminate between connections. In practice, the goal was to
    try to detect certain dead connections but all cases leading to
    such connections are either already handled in the tests before
    (which don't reach this label), or are handled by the other
    conditions.

Thus, let's remove this confusing test.