overlay: Remove back button panic overlay

* Google decided to kill the feature.
   See the related fw/base commit for reference:
   bb97879c95

Change-Id: Iaa85020c4b2ab69d4e7cf7415e04e08990a9ea4d

overlay/common/frameworks/base/core/res/res/values/config.xml

@@ -67,12 +67,6 @@
          non-interactive state. -->
     <bool name="config_supportLongPressPowerWhenNonInteractive">true</bool>
 
-    <!-- Control the behavior when the user panic presses the back button.
-            0 - Nothing
-            1 - Go to home
-    -->
-    <integer name="config_backPanicBehavior">1</integer>
-
     <!--  Whether Multiuser UI should be shown -->
     <bool name="config_enableMultiUserUI">true</bool>