Removed macOS native code which broke the sketch plugin in newer macOS versions

[oliverschwendener]

This PR fixes some bugs which made the plugin crash when choosing brand assets.

• Additionally removed fallbacks for Sketch version 68 and older.

[jonasbuechel]

Tested successfully on 2016 Macbook with and 2021 Macbook (M1) both using Sketch 84 (140208)

• authentication to weare.frontify.com
• replacing colors directly for texts and reactangles
• adding / replacing color palettes to document
• adding / replacing brandassets: images, logos
• uploading artboards

As I see we alsofixed using colors directly from the plugins (without adding them to palettes) for non-M1 :)