fix: Add missing 'rembg' dependency

[GennaroBaratta]

What does this PR do?

This pull request adds the rembg package to the requirements.txt file.

Why is this change necessary?

On a fresh installation of this custom node, ComfyUI fails to load it and throws a ModuleNotFoundError: No module named 'rembg' in the console.

The rembg library is a direct dependency for the background removal functionality but was not declared in the project's requirements. This change ensures that users can automatically install all necessary dependencies via pip, preventing the node from failing to load.

How have you tested this?

1. Set up a clean ComfyUI virtual environment.
2. Cloned the repository and tried to start ComfyUI.
3. Result (Before): Confirmed the ModuleNotFoundError appeared and the nodes were not loaded.
4. Applied this change (added rembg to requirements.txt).
5. Ran pip install -r requirements.txt from within the node's directory.
6. Result (After): Restarted ComfyUI, the error was gone, and the nodes loaded successfully.

This resolves the installation issue for new users.