Remove deprecated platforms from README

[jcesarmobile]

We have removed the code for deprecated platforms, but we still mention them as supported and it's quirks, we should remove all that.

[janpio]

(You can probably duplicate this in apache/cordova and add a TODO for all plugins...)

[jcesarmobile]

I should check all plugins first, maybe they were removed correctly in other plugins

[janpio]

maybe...

[jcesarmobile]

In fact it was correctly removed, but then added by accident later, probably because of the automatic doc generation, I've only checked a few other repos but didn't have the deprecated platforms anymore.