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.

tests/test-clone-stream.t

@@ -683,40 +683,6 b' Stream clone while repo is changing:'
   $ mkdir changing
   $ cd changing
 
-extension for delaying the server process so we reliably can modify the repo
-while cloning
-
-  $ cat > stream_steps.py <<EOF
-  > import os
-  > import sys
-  > from mercurial import (
-  >     encoding,
-  >     extensions,
-  >     streamclone,
-  >     testing,
-  > )
-  > WALKED_FILE_1 = encoding.environ[b'HG_TEST_STREAM_WALKED_FILE_1']
-  > WALKED_FILE_2 = encoding.environ[b'HG_TEST_STREAM_WALKED_FILE_2']
-  > 
-  > def _test_sync_point_walk_1(orig, repo):
-  >     testing.write_file(WALKED_FILE_1)
-  > 
-  > def _test_sync_point_walk_2(orig, repo):
-  >     assert repo._currentlock(repo._lockref) is None
-  >     testing.wait_file(WALKED_FILE_2)
-  > 
-  > extensions.wrapfunction(
-  >     streamclone,
-  >     '_test_sync_point_walk_1',
-  >     _test_sync_point_walk_1
-  > )
-  > extensions.wrapfunction(
-  >     streamclone,
-  >     '_test_sync_point_walk_2',
-  >     _test_sync_point_walk_2
-  > )
-  > EOF
-
 prepare repo with small and big file to cover both code paths in emitrevlogdata
 
   $ hg init repo