Remove imagen_r2i and nano_banana from copybara open source file list

Removed the following paths from open_source_projects_list:
- projects/ai/gen-media/notebooks/imagen_r2i/**
- projects/ai/gen-media/notebooks/nano_banana/**

These notebooks are not intended for open source release.

Change-Id: I97c32f3e5bdd69287d674b2cb2679321e9494362
GitOrigin-RevId: 5c052604e51ddc03e1080d15908babf80dd63bb2

Author: layolin