Remove deprecated `fractionalEvaluation` operator

The `fractionalEvaluation` operator has been replaced with `fractional` and can be removed.

[This Go file](https://github.com/open-feature/flagd/blob/2f330af516376238e889064500a285b13ce8bb9a/core/pkg/evaluator/legacy_fractional.go#L25) and its references can be deleted.