Remove targetDocument autocompletion

[malarzm]

This dodges a BC break, probably not impacting a lot of users but still a BC break. I use single reference as an example but same goes for EmbedOne. Currently

    /** @ODM\ReferenceOne() */
    private BaseDocument $referenceToAnywhere;

is a perfectly fine mapping that allows user to have a reference to any document (from ODM's perspective) as long as it extends BaseDocument (from PHP's perspective). With #2411 due to autocompletion for typed properties this would become (implicitly):

    /** @ODM\ReferenceOne(targetDocument=BaseDocument::class) */
    private BaseDocument $referenceToAnywhere;

which is wrong. This could be worked around by an explicit targetDocument=false in the mapping but that would then require us to widen targetDocument to allow |false. As we want to get 2.4.0 out of the door let's defer the decision and work to future us.

If we ever get to implementing this feature there's a number of edge cases we'll need to consider as well. All I've found so far involve having typed properties and discriminators (be it maps or anything else hinting discriminators may be in use). Hopefully these will be easier to spot with my next PR which will move autocompleting types before some mapping sanity checks.