[breaking] Remove the predictor param, allow fallback to prediction using DMatrix. (#9129)

- A `DeviceOrd` struct is implemented to indicate the device. It will eventually replace the `gpu_id` parameter.
- The `predictor` parameter is removed.
- Fallback to `DMatrix` when `inplace_predict` is not available.
- The heuristic for choosing a predictor is only used during training.

tests/python/test_with_sklearn.py

@@ -1418,23 +1418,6 @@ def test_categorical():
     np.testing.assert_allclose(predt_cat, predt_enc)
 
 
-def test_prediction_config():
-    reg = xgb.XGBRegressor()
-    assert reg._can_use_inplace_predict() is True
-
-    reg.set_params(predictor="cpu_predictor")
-    assert reg._can_use_inplace_predict() is False
-
-    reg.set_params(predictor="auto")
-    assert reg._can_use_inplace_predict() is True
-
-    reg.set_params(predictor=None)
-    assert reg._can_use_inplace_predict() is True
-
-    reg.set_params(booster="gblinear")
-    assert reg._can_use_inplace_predict() is False
-
-
 def test_evaluation_metric():
     from sklearn.datasets import load_diabetes, load_digits
     from sklearn.metrics import mean_absolute_error