odb: drop deprecated wrapper functions

In the Git 2.51 release cycle we've refactored the object database layer to access objects via struct object_database directly. To make the transition a bit easier we have retained some of the old-style functions in case those were widely used.

Now that Git 2.51 has been released it's time to clean up though and drop these old wrappers. Do so and adapt the small number of newly added users to use the new functions instead.

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