Make txn_two_phase_commit_killop.js not kill prepareTransaction on the coordinator

The way that the test finds the pids for the operations to kill is through a $currentOp aggregation command with query {desc: "TransactionCoordinator"}. The command can return the pid for the prepareTransaction operation on the coordinator shard itself. Killing that operation is expected to cause the transaction to abort. The currentOp entry for the prepareTransaction operation should have the command obj inside the "command" so we can make use of this property to filter out its pid.