fix: simplifying gemini all modalities sample

Description

Fixes #

Note: Before submitting a pull request, please open an issue for discussion if you are not associated with Google.

[ ] I have followed Contributing Guidelines from CONTRIBUTING.MD
[ ] Tests pass: go test -v ./.. (see Testing)
[ ] Code formatted: gofmt (see Formatting)
[ ] Vetting pass: go vet (see Formatting)
[ ] These samples need a new API enabled in testing projects to pass (let us know which ones)
[ ] These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
[ ] This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
[ ] This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
[ ] Please merge this PR for me once it is approved