[Mono.Android] Remove enum mapping for DisplayMetrics.DENSITY_DEVICE_STABLE

Commit 50c0c15 [broke][0] the [build][1]:

[0]: https://jenkins.mono-project.com/view/Xamarin.Android/job/xamarin-android/784/consoleText
[1]: https://jenkins.mono-project.com/view/Xamarin.Android/job/xamarin-android/784/

	obj/Release/android-27/mcw/Android.Util.DisplayMetricsDensity.cs(22,20): error CS0150: A constant value is expected

Remove the enum mapping for DisplayMetrics.DENSITY_DEVICE_STABLE so
that the build is no longer broken.

Author: jonpryor

src/Mono.Android/map.csv

@@ -4535,7 +4535,6 @@
 23,Android.Util.DisplayMetricsDensity,D420,android/util/DisplayMetrics.DENSITY_420,420
 21,Android.Util.DisplayMetricsDensity,D560,android/util/DisplayMetrics.DENSITY_560,560
 10,Android.Util.DisplayMetricsDensity,Default,android/util/DisplayMetrics.DENSITY_DEFAULT,160
-24,Android.Util.DisplayMetricsDensity,DeviceStable,android/util/DisplayMetrics.DENSITY_DEVICE_STABLE,
 10,Android.Util.DisplayMetricsDensity,High,android/util/DisplayMetrics.DENSITY_HIGH,240
 10,Android.Util.DisplayMetricsDensity,Low,android/util/DisplayMetrics.DENSITY_LOW,120
 10,Android.Util.DisplayMetricsDensity,Medium,android/util/DisplayMetrics.DENSITY_MEDIUM,160