[FIX] OWScatterPlot: Fix crash on classless data

[VesnaT]

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[codecov-io]

Current coverage is 87.91%

Merging #1440 into master will not change coverage

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[lanzagar]

I think at first the information message was meant for the VizRank widget. After some refactoring it is now a more lightweight QDialog and can't have warnings anymore.

But using the same message on the main ScatterPlot widget is not OK.
"Data with a class variable is required." is wrong and confusing there.

An acceptable half-solution would be to use a better message, e.g. "Automatic scoring requires data with a class variable".
Even better in my opinion is to not use the information mechanism at all, and just make the message a tooltip on the disabled button.