Clean up stray monochrome flags.

The monochrome flags have been removed
from both the encoder and decoder, as this
is now an option in the colour space.

Change-Id: Iccd0f051f35811174ba286c6ee454443fd5268be
diff --git a/examples/encoder_util.h b/examples/encoder_util.h
index 72057a9..38deef0 100644
--- a/examples/encoder_util.h
+++ b/examples/encoder_util.h
@@ -31,6 +31,6 @@
 
 // Returns 1 if the two images match.
 int aom_compare_img(const aom_image_t *const img1,
-                    const aom_image_t *const img2, int num_planes);
+                    const aom_image_t *const img2);
 
 #endif  // EXAMPLES_ENCODER_UTIL_H_