Gut ash::MaterialDesignController, and remove the about:flags entry.

chrome/app/generated_resources.grd

Index: chrome/app/generated_resources.grd
diff --git a/chrome/app/generated_resources.grd b/chrome/app/generated_resources.grd
index 3a1cd6bd94cf72353e6ce8ae0205a3eff8b68a24..0cb8b9e51887730ff677b0c6cdf35156ff7dd50a 100644
--- a/chrome/app/generated_resources.grd
+++ b/chrome/app/generated_resources.grd
@@ -5589,15 +5589,6 @@ Keep your key file in a safe place. You will need it to create new versions of y
           </message>
       </if>
       <if expr="use_ash">
-        <message name="IDS_FLAGS_ASH_MD" desc="Title of the flag which enables or disables material design in Chrome OS system UI.">
-          Material Design in Chrome OS system UI
-        </message>
-        <message name="IDS_FLAGS_ASH_MD_DESCRIPTION" desc="Description of the flag which changes the material design elements in Chrome OS system UI.">
-          Sets the Material Design elements in Chrome OS system UI.
-        </message>
-        <message name="IDS_FLAGS_ASH_MD_EXPERIMENTAL" desc="The value of the Material Design in Chrome OS UI flag that controls experimental features.">
-          Experimental
-        </message>
         <message name="IDS_FLAGS_ASH_MAXIMIZE_MODE_WINDOW_BACKDROP_NAME" desc="Title for the flag which can be used for window backdrops in TouchView.">
           Window backdrops in TouchView
         </message>