oplog_format_create_indexes.js should ignore 'ns' field in multi version tests

We removed the 'ns' field from the index specs in SERVER-41696 and updated oplog_format_create_indexes.js accordingly. However, this test may be run with a mixed version replica set containing 4.2 binaries that still support the 'ns' field. This test should be modified, similar to what's currently done in the 4.2 branch, to ignore the 'ns' field to fix the test failures in the CI system