Drop cordova-osx support

[BBosman]

Feature Request

Motivation Behind Feature

With the deprecation of cordova-osx as an officially supported platform it seems a logical step to also deprecate its support within this plugin.

Feature Description

Remove <platform name="osx"> and related files.

Alternatives or Workarounds

n/a

Next steps

If this is a wanted change, I'm willing to create a PR for it.

[breautek]

Something like this is generally looked at when we are ready to start preparing a new major release.

Because I don't think we have a reason to introduce a new major release anytime soon, I created a milestone to help us track this so it doesn't become forgotten/missed.