Not able to delete the key from layer settings using Settings.setLayerSettingForKey
The key should be deleted from the layer user info by passing `undefined` in the `Settings.setLayerSettingForKey(layer, 'my-key', undefined)` but the key is not removed and the value is set to null.

May be implement `removeObjectForKey(key)` when the value is undefined at

https://github.com/BohemianCoding/SketchAPI/blob/cea79d8b9b37f2c21064f7ddd310eaa5862bda7d/Source/settings/Settings.js#L116 

Ref: https://sketchplugins.com/d/1433-how-to-delete-a-layer-setting