hide label of _delete in AdminType inline table

before:
![image](https://cloud.githubusercontent.com/assets/241080/2958269/d77954ac-daa7-11e3-8292-0489378bf44d.png)

after:
![image](https://cloud.githubusercontent.com/assets/241080/2958261/ba5f69f6-daa7-11e3-84ed-b739460b9109.png)