Further restrict unused expressions

See: [no-unused-expressions](https://eslint.org/docs/rules/no-unused-expressions)
See: https://github.com/MetaMask/eslint-config/blob/108d224e48fdf7f6f05de2bf2ece1fe65484c89b/config/index.js#L208

All unused expressions should be disabled.

Open config/index.js at the referenced line and read the no-unused-expressions rule documentation linked in the issue. Confirm how the shared ESLint configuration currently handles unused expressions, then verify the configuration enforces the requested restriction and run the repository's lint checks to confirm the change.

Written by the indexing model from the issue text.