Remove last MD reference from TrayPopupUtils.

ash/common/system/tray/tray_constants.h

Index: ash/common/system/tray/tray_constants.h
diff --git a/ash/common/system/tray/tray_constants.h b/ash/common/system/tray/tray_constants.h
index 87f6175bcdb97756c8f3209362980e99e31b1333..1da6f92bfc20dc8b7ee73ce422533bf52e0841fd 100644
--- a/ash/common/system/tray/tray_constants.h
+++ b/ash/common/system/tray/tray_constants.h
@@ -52,10 +52,6 @@ extern const int kTrayPopupLabelHorizontalPadding;
 // before the transition starts.
 ASH_EXPORT extern const int kTrayDetailedViewTransitionDelayMs;
 
-// Padding used to adjust the slider position in volume row and brightness
-// row horizontally.
-extern const int kTrayPopupSliderPaddingMD;
-
 // Padding used on right side of labels to keep minimum distance to the next
 // item. This applies to all labels in the system menu.
 extern const int kTrayPopupLabelRightPadding;