MAINT: _validate_line_lengths redefined in FASTAFormat #393

It seems like the _validate_line_lengths method is superfluously defined in the FASTAFormat class. It is also defined in the AlignedFASTAFormatMixin class. It is only called in the validator of FASTAFormat if self.aligned, which only AlignedFASTAFormatMixin has the ability to set. So it seems like it should only live in AlignedFASTAFormatMixin.