How to remove shadow between tool bar and tab layout

Question

How do I remove the shadow bellow the toolbar layout but keep the shadow below the tab layout... I've tried many ways but can't find the answer on SO. Please help.

note app:elevation="0dp" has no effect on Toolbar layout shadow

<android.support.design.widget.CoordinatorLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <android.support.design.widget.AppBarLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar">

               <android.support.v7.widget.Toolbar
                   android:id="@+id/toolbar"
                   android:layout_width="match_parent"
                   android:layout_height="?attr/actionBarSize"
                   android:background="?attr/colorPrimary"
                   app:layout_scrollFlags="scroll|enterAlways"
                   app:popupTheme="@style/ThemeOverlay.AppCompat.Light"
                   />

                <android.support.design.widget.TabLayout
                    android:id="@+id/tabs"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    app:tabMode="fixed"
                    app:tabGravity="fill"/>

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

    <android.support.v4.view.ViewPager
        android:id="@+id/viewpager"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_behavior="@string/appbar_scrolling_view_behavior"  />

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