chore: Remove unused logo image (#202)

To improve maintainability and reduce redundancy, this PR deletes the local logo file. The logo displayed in all READMEs (top-level and subpackages) is already sourced from the `googleapis/genai-toolbox` repository: [link to logo](https://github.com/googleapis/genai-toolbox/blob/main/logo.png).

Consolidating on the server repository's logo as the single source of truth will make future updates easier.

> [!NOTE]
> We will continue to source logos and favicons, including for potential future uses like a docsite, from the `googleapis/genai-toolbox` server repository.

Author: anubhav756