The `LayerGroup` class had an API to add layers and layer groups.
The only thing these did was allocate a new layer/group and then
call the `add_node` API.
This removes these functions (they were only used in a single
place anyway) and does the allocation of the new layer/group
directly in the grease pencil API. This removes a few lines of
code and makes everything a bit easier to read.
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