Code review comment for lp:~mir-team/mir/repair-client-abi

As noted in https://code.launchpad.net/~afrantzis/mir/platform-operation-client-api-remove-opcode/+merge/248338 :

"This is a client API/ABI break, but since the platform operation API is not currently used by external code (and could not have been used because we didn't expose any operations), we can get away with it."

However, since this fix has already landed independently on 0.11, it's good to be in sync.