(bug 62644) No snak val in ClaimModificationHelper

Now that snak validation takes place in ChangeOps,
don't do it in ClaimModificationHelper any more.

Note: this makes SnakValidationHelper obsolete, but
we keep it around to be refactor into a generic ApiErrorReporter
later.

DEPLOYMENT: The API error code used to report invalid snaks
(and bad operations in ChangeOps in general) is now
"modification-failed". More specific error codes may be introduced
later.

Change-Id: Ieb422a8f340c21db85d849d55613ceb96fe24f50
Merge "(bug 62644) No snak val in ClaimModificationHelper"