[Lens] Change toolbar popovers' disabled form rows to hidden instead

[MichaelMarcialis]

Per my previous comments, seeing as the settings within the Lens toolbar popovers continue to grow more robust and complex, I think now is a good time to revisit my previous suggestion to "disable form fields when the user has the ability to enable them in the current context." While that approach works in simpler situations (where there is a desire to showcase all available features to users), I worry that it is now becoming too cluttered and visually noisy for the user to be presented with so many disabled fields that they have to stumble past.

Can we transition the form rows that are currently being disabled in these toolbar popovers from being disabled to now be hidden instead, only showing them when they available (as we've begun doing in the configuration flyouts, per the latest annotation designs)?

[elasticmachine]

Pinging @elastic/kibana-vis-editors @elastic/kibana-vis-editors-external (Team:VisEditors)