Fix a comment from "Speed up mv cost table setup"

Chi Yo Tsai noticed this comment in code review, but the patch was
merged before I could fix the comment. The original comment did not make
sense and looked like a stray line.

Change-Id: Ib1b50543fae0386e4104450a41227309e8960739
diff --git a/av1/encoder/encodemv.c b/av1/encoder/encodemv.c
index a03cdfa..7cae72c 100644
--- a/av1/encoder/encodemv.c
+++ b/av1/encoder/encodemv.c
@@ -220,7 +220,7 @@
         mvcost[-v] = cost + negate_sign;
       }
     }
-    // t = ((2 * MV_FP_SIZE) << MV_OFFSET_BITS) - 1
+    // At this point: mantissa = exponent >> 1
 
     // Manually calculate the final cost offset
     int cost_swap_hi =