Remove evaluation functions returning errors

> [Suggested by @toddbaert] We could consider removing [these functions](https://github.com/open-feature/go-sdk/blob/bf2d408e2945f2de67a0d3e608adb630df5c0881/openfeature/interfaces.go#L30-L34) entirely. They return errors directly, which add complexity and is arguably against the spec, which says evaluation methods should never abnormally terminate (returning a non-nil error in Go signals an abnormal termination equivalent to throwing IMO). People actually complained about the ergonomics of these and we implemented the [non-error-returning versions in response](https://github.com/open-feature/go-sdk/blob/bf2d408e2945f2de67a0d3e608adb630df5c0881/openfeature/interfaces.go#L41-L45). The "detailed" evaluation methods also return errors, and we have no "non-error" variant. We should consider our options here to make these consistent and idiomatic.

Consider removing evaluation functions returning errors. This is somewhat against spec, and people complained about the ergonomics long ago, causing us to build the non-error-returning versions.