How can I remove the border around the toolbar?

In my app I'm using the toolbar and it seems I want to get rid of the border around it:

Then I create it like this:

<android.support.design.widget.AppBarLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:background="@color/transparent">

    <android.support.v7.widget.Toolbar
        android:id="@+id/toolbar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center"
        android:layout_gravity="center"
        android:minHeight="?attr/actionBarSize"/>

    </android.support.design.widget.AppBarLayout>

</android.support.design.widget.CoordinatorLayout>

I use it like this:

<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:orientation="vertical">

    <include
        layout="@layout/app_bar_main_light"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />
</LinearLayout>

And I never found a property or style that specifies what this border is.