Remove bogus step from test_decoding isolation tests.
Commit
43b4a16817c8b5568cec72f3b0e1c8209f5ac7f7 made the isolation
tester reject duplicate step names, and it turns out that the
test_decoding module's concurrent_ddl_dml isolation test has a
duplicate name. I think the second definition isn't actually getting
used, so just remove it.
Per buildfarm.