Remove superfluous `if` condition in Model

The method `Model::fromDateTime()` method has an if/else chain that terminates in a statement that will always evaluate to true given the control flow. The first if condition in the chain is `if ($value instanceof DateTime)`, while the final elseif condition is `elseif ( ! $value instanceof DateTime)`. Since the first condition must be true to arrive at the final check, the negation will always be true at that point, so it is a superfluous condition.