Removed redundant code in detachBehaviors()

After ensureBehaviors() call $this->_behaviors is an array already.
And since detachBehavior() unsets elements in $this->_behaviors, $this->_behaviors turns out to be an empty array after all iterations.

framework/yii/base/Component.php

@@ -543,13 +543,10 @@ class Component extends Object
 	public function detachBehaviors()
 	{
 		$this->ensureBehaviors();
-		if ($this->_behaviors !== null) {
 		foreach ($this->_behaviors as $name => $behavior) {
 			$this->detachBehavior($name);
 		}
 	}
-		$this->_behaviors = [];
-	}
 
 	/**
 	 * Makes sure that the behaviors declared in [[behaviors()]] are attached to this component.