refactor: remove redundant Begin() calls after NewSession auto-begins

Since NewSession() now auto-begins a transaction, explicit Begin()
calls are redundant (xorm's Begin() is a no-op when already in a
transaction). Removing them reduces confusion.

Special case: user_delete.go's loop previously called Begin/Commit
per user on a shared session. Restructured to create a new session
per user deletion so each gets its own transaction.

pkg/cmd/user.go

@@ -369,10 +369,6 @@ var userDeleteCmd = &cobra.Command{
 		s := db.NewSession()
 		defer s.Close()
 
-		if err := s.Begin(); err != nil {
-			log.Fatalf("Count not start transaction: %s", err)
-		}
-
 		u := getUserFromArg(s, args[0])
 
 		if userFlagDeleteNow {