[Lens] Fitting functions are not cleared when passing from Line to Area charts

[dej611]

Kibana version: master on fe1ae92 (> 7.12)

Describe the bug:

Fitting functions are currently disabled for area percentages (it is enabled for both area and area stacked charts, even when using date histograms - perhaps the message shown is wrong?) as there's no fitting to apply.

It is still possible to enabled it in area percentages passing by other types of chart, setting the fitting function, then changing the chart again to area percentage.
This should not an issue in general, but because #93648 it became visible.

Screenshots (if relevant):

Once #93648 gets resolved the actual problem will disappear, but I think it's still worth cleaning up a bit the state.
Also, the message should be improved in this scenario

[elasticmachine]

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

[dej611]

I think we can close this as #94086 sets a fixed value as the fitting function in this case.