scale: little API change in linear scale

Follow-up of PR #3753. The linear scale, LinearScale, has an empty constructor that swallows **kwargs. I propose to dump it.

(LogScale and its symmetric twin are a different case, because they use **kwargs to discriminate x axes from y axes.)
wait: why don't we include this dummy constructor in the `ScaleBase` class then? Every to-be-registered scale needs to accept those arguments because of the `scale_factory` function...