Improve comments in RangeImage

Also remove some dead code.

Change-Id: Ia0635e7b47dd52ef72b673ee4272c11a86f9ba2c
diff --git a/aos/vision/blob/range_image.cc b/aos/vision/blob/range_image.cc
index 613d2ca..81a8c48 100644
--- a/aos/vision/blob/range_image.cc
+++ b/aos/vision/blob/range_image.cc
@@ -5,18 +5,10 @@
 
 namespace aos {
 namespace vision {
+namespace {
 
-void DrawRangeImage(const RangeImage &rimg, ImagePtr outbuf, PixelRef color) {
-  for (int i = 0; i < (int)rimg.ranges().size(); ++i) {
-    int y = rimg.min_y() + i;
-    for (ImageRange rng : rimg.ranges()[i]) {
-      for (int x = rng.st; x < rng.ed; ++x) {
-        outbuf.get_px(x, y) = color;
-      }
-    }
-  }
-}
-
+// Merge sort of multiple range images into a single range image.
+// They must not overlap.
 RangeImage MergeRangeImage(const BlobList &blobl) {
   if (blobl.size() == 1) return blobl[0];
 
@@ -48,6 +40,8 @@
   }
 }
 
+}  // namespace
+
 std::string ShortDebugPrint(const BlobList &blobl) {
   RangeImage rimg = MergeRangeImage(blobl);
   std::string out;