Remove gdx exporting from spine_io

[soininen]

The module can be removed now that we've dropped the GdxExporter project item from spine_items.

Re spine-tools/Spine-Toolbox#1263

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [x] Release notes in Toolbox repo have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass