remove set_prefix (#219)

See:
- deepmodeling/deepmd-kit#3477
- deepmodeling/deepmd-kit#3753

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Refactor**
	- Removed an unused configuration parameter from the training settings.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

tests/op/test_prep_dp_train.py

@@ -34,7 +34,6 @@
     "model": {"descriptor": {"type": "se_e2_a", "seed": 1}, "fitting_net": {"seed": 1}},
     "training": {
         "systems": [],
-        "set_prefix": "set",
         "stop_batch": 2000,
         "batch_size": "auto",
         "seed": 1,
@@ -51,7 +50,6 @@
     },
     "training": {
         "systems": [],
-        "set_prefix": "set",
         "stop_batch": 2000,
         "batch_size": "auto",
         "seed": 1,