Remove comment for ImageDecoder::FilenameExtension

Remove the comment for the ImageDecoder::FilenameExtension() method that
I added in commit 8d8bd398.

Change-Id: I1f64e14d06bb934af1c5b8822212158d5db0b4b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2219593Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773480}

third_party/blink/renderer/platform/image-decoders/image_decoder.h

@@ -178,8 +178,6 @@ class PLATFORM_EXPORT ImageDecoder {
                   color_behavior, allow_decode_to_yuv, desired_size);
   }
 
-  // The return value of FilenameExtension() is often used as a string
-  // identifier of the image type.
   virtual String FilenameExtension() const = 0;
 
   bool IsAllDataReceived() const { return is_all_data_received_; }