Simplify mirror repository API logic (#30963)

Fix #30921

templates/swagger/v1_json.tmpl

@@ -20753,7 +20753,7 @@
           "x-go-name": "Description"
         },
         "enable_prune": {
-          "description": "enable prune - remove obsolete remote-tracking references",
+          "description": "enable prune - remove obsolete remote-tracking references when mirroring",
           "type": "boolean",
           "x-go-name": "EnablePrune"
         },