Refactor. (#4612)

### What problem does this PR solve?

### Type of change

- [x] Refactoring

api/db/services/user_service.py

@@ -50,12 +50,6 @@ class UserService(CommonService):
         else:
             return None
 
-    @classmethod
-    @DB.connection_context()
-    def user_gateway(cls, tenant_id):
-        hashobj = hashlib.sha256(tenant_id.encode("utf-8"))
-        return int(hashobj.hexdigest(), 16)%len(MINIO)
-
     @classmethod
     @DB.connection_context()
     def save(cls, **kwargs):
@@ -134,6 +128,12 @@ class TenantService(CommonService):
         if num == 0:
             raise LookupError("Tenant not found which is supposed to be there")
 
+    @classmethod
+    @DB.connection_context()
+    def user_gateway(cls, tenant_id):
+        hashobj = hashlib.sha256(tenant_id.encode("utf-8"))
+        return int(hashobj.hexdigest(), 16)%len(MINIO)
+
 
 class UserTenantService(CommonService):
     model = UserTenant