stream-clone: remove unused code in test-clone-stream.t
We are not using the extension we create inline, we are using
`tests/testlib/ext-stream-clone-steps.py`. So let us deleted the unused version.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}