[Lens] Don't show x-dimension required when not required

[timroes]

This is a follow up from an offline discussion @dej611 and I had.

Currently in Lens, when you just drag a time field into the y-axes we'll be left with an error stating the x-axes is required, but still draw the chart:

I think we should not show that error while we're able to draw a chart, since it obviously means the dimension is not really required. I understand there are some edge-cases and this issue should be to outline those and come to a conclusion whether we want to show that error or not in those scenarios.

cc @flash1293

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[flash1293]

I agree with you, @timroes - there is merit in pointing the user to complete the chart by adding an x axis, but I think in this case it causes more confusion than being helpful. IMHO users will get what to do even without pointing them there using an error message. The suggestion logic will still try hard to have an x axis in the chart in most scenarios (which is a good thing I think).

@wylieconlon As this was added as part of the percentage charts based on your comment (#70703 (comment)), would you be fine with removing the message?

[wylieconlon]

@flash1293 fine with me