Remove unneccessary conditional
Remove unneccessary conditional

When `messageValues` is a `Function` it always returns an object, as it
is one of these functions:

* `lib/actual-and-expected-message-values.js`
* `lib/actual-and-type-of-message-values.js`
* `lib/actual-for-match.js`
* `lib/actual-message-values.js`

busterjs/referee