Remove extra DNG images and use smaller HTC.dng

Update SK_IMAGE_VERSION to 7. This removes the following files from
dm testing:

lg_g4_iso_800.dng
Fuji_X20.dng

These DNG images are very large, and cause us to use a lot of memory in
testing. Moreover, they do not test anything that is not covered by
HTC.dng

In addition, use the smaller version of HTC.dng (which is already in
use in dm) in nanobench.

BUG=skia:5307
BUG=skia:5283
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2007033002

Review-Url: https://codereview.chromium.org/2007033002
diff --git a/SK_IMAGE_VERSION b/SK_IMAGE_VERSION
index 1e8b314..7f8f011 100644
--- a/SK_IMAGE_VERSION
+++ b/SK_IMAGE_VERSION
@@ -1 +1 @@
-6
+7