duplicated initDefaultButtons() call in ActionColumn

[santilin]

Prerequisites

• [ x ] I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• [ x ] The issue still exists against the latest master branch of yii2-grid.
• [ x ] This is not an usage question. I confirm having gone through and read the documentation and demos.
• [ x ] This is not a general programming / coding question. (Those should be directed to the webtips Q & A forum).

On ActionColumn::init() there is a call to initDefaultButtons() which is already done in Yii2::ActionColumn::init()

I think the call in kartik's code should be removed.