get rid of HAL pixelformats 5551 and 4444

Change-Id: I047d948f2f3b5c681a0b187589775f92b4f01541
diff --git a/include/system/graphics.h b/include/system/graphics.h
index 2ddcc08..5d883be 100644
--- a/include/system/graphics.h
+++ b/include/system/graphics.h
@@ -54,8 +54,6 @@
     HAL_PIXEL_FORMAT_RGB_888            = 3,
     HAL_PIXEL_FORMAT_RGB_565            = 4,
     HAL_PIXEL_FORMAT_BGRA_8888          = 5,
-    HAL_PIXEL_FORMAT_RGBA_5551          = 6,
-    HAL_PIXEL_FORMAT_RGBA_4444          = 7,
 
     /*
      * sRGB color pixel formats: