Simplify mirror repository API logic (#30963) (#31009)

Backport #30963 by wxiaoguang

Fix #30921

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/swagger/v1_json.tmpl

@@ -20610,7 +20610,7 @@
           "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"
         },