Remove --lock warning when it's not passed

Previously you would *always* get the deprecation warning. Regardless if
it was passed.

Note however that it still can have an effect to pass the flag.
Unfortunately, the hash of the lock-file is based on even the
non-semantical flags like --lock (I want to kill myself).
Merge pull request #374 from Tarrasch/remove-lock-warning

Remove --lock warning when it's not passed