Revert "image_signing: use sparse partition extraction [reland]"

This reverts commit f482470b72.

The original change is the suspected root cause of chromium:606738.

TEST=signing_unittests.py in cros-signing passes (with updated
vboot_stable_hash).
BUG=chromium:606738
BRANCH=None

Change-Id: I21ea027bbda123ee26c6deb4437b07d2fc6e8575
Reviewed-on: https://chromium-review.googlesource.com/340895
Commit-Ready: Amey Deshpande <ameyd@google.com>
Tested-by: Amey Deshpande <ameyd@google.com>
Reviewed-by: Amey Deshpande <ameyd@google.com>

scripts/image_signing/common_minimal.sh

@@ -246,7 +246,7 @@ extract_image_partition() {
   local offset=$(partoffset "$image" "$partnum")
   local size=$(partsize "$image" "$partnum")
   dd if=$image of=$output_file bs=512 skip=$offset count=$size \
-    conv=notrunc,sparse 2>/dev/null
+    conv=notrunc 2>/dev/null
 }
 
 # Replace a partition in an image from file