workspace validator: suppress notices about download

[bertsky]

When I run workspace validation these days, I get a lot of these in the final report:

<notice>_validate_dimension: Not executed because --download wasn't set and PAGE might reference remote (Alternatve)Images <...></notice>

IMHO these do not belong in the report at all. If anything, they should be output as warning-level messages in the log during validation.

But to me this is not even justified in the first place: ocrd_validators.workspace_validator.WorkspaceValidator._validate_dimension never actually checks whether the PAGE file exists in the filesystem.

And the problem definitely does not extend to AlternativeImage. (So at least that part of the notice is unwarranted.)