Don't warn about file group syntax

[kba]

I.e.

WARNING: input fileGrp 'MAX' does not conform to OCR-D spec
WARNING: output fileGrp 'WOLF,WOLF-IMG' does not conform to OCR-D spec

should be gone. Makes no sense to warn about reality not conforming to our specs.

[kba]

Actually, I think we should not validate for group name syntax in core at all or at least make this a warning and relax the regexp. It's annoying and pointless.

[kba]

https://github.com/OCR-D/core/releases/tag/v2.12.0