get rid of PIXEL_FORMAT_A_8

Change-Id: I4e409a69c01b95b48f20112084dd927e2d043407
diff --git a/include/ui/PixelFormat.h b/include/ui/PixelFormat.h
index 9f3e267..7b3a6de 100644
--- a/include/ui/PixelFormat.h
+++ b/include/ui/PixelFormat.h
@@ -63,7 +63,6 @@
     PIXEL_FORMAT_BGRA_8888   = HAL_PIXEL_FORMAT_BGRA_8888,  // 4x8-bit BGRA
     PIXEL_FORMAT_RGBA_5551   = HAL_PIXEL_FORMAT_RGBA_5551,  // 16-bit ARGB
     PIXEL_FORMAT_RGBA_4444   = HAL_PIXEL_FORMAT_RGBA_4444,  // 16-bit ARGB
-    PIXEL_FORMAT_A_8         = 8,                           // 8-bit A
 };
 
 typedef int32_t PixelFormat;