remove set_prefix

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

Author: njzjz

source/CaseStudies/Mg-Y_alloy/Mg-Y_alloy.md

@@ -2661,7 +2661,6 @@ The examplory files are attached: init_bulk param.json, run param.json. machine-
 
     "training" : {
      "systems":          [],
-     "set_prefix":       "set",
      "stop_batch":       600000,
      "batch_size":       1,
 

source/CaseStudies/Transfer-learning/Transfer-learning.md

@@ -114,7 +114,6 @@ Examplary "simplify.json" of Ag-Au system:
   "_comment": " traing controls",
   "training" : {
             "systems":      [],
-            "set_prefix":   "set",
             "stop_batch":   400000,
             "batch_size":   2,
             "seed":         1,