chore: set sample-snap as private

[mikesposito]

Explanation

packages/sample-snap was initially set as private: false to comply with several CI jobs that required at least one package to be in the monorepo to properly work.

As the @metamask/bitcoin-wallet-snap package migration has been completed, we can now set it to private: true.

References

N/A

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them