Unexport and undocument LogoControl

[jfirebaugh]

Other than the logoPosition map option, there's no public API -- it's shown/hidden automatically.

[mollymerp]

It's not yet shown/hidden automatically for all users because there are still remaining API changes that are blocked. Some users still need to add the logo manually, but if we want to make the Control private and just have them use the HTML snippet to add it as has been the practice up til now that's fine too.

@jfirebaugh I think there's also a line in documentation.yml for LogoControl that should be removed if we do decide to make it completely private

[jfirebaugh]

Some users still need to add the logo manually

I assumed this wasn't possible/needed because all instances of LogoControl are going to be either shown or hidden based on what _updateLogo() returns. So if a user added one manually in addition to the automatic one, there would be either none visible still, or two visible. Do we need to handle this some other way?

[mollymerp]

Yep, you're right about _updateLogo -- I'm good with making this private because the API changes should ship soon so it might not be worth it to make an alternative in the meantime.

LGTM once https://github.com/mapbox/mapbox-gl-js/blob/unexport-logocontrol/documentation.yml#L21 is also removed.