CCA: Put barcode scan box shadow under the settings menu

The z-index on scan box made the box-shadow covers the settings menu, so
the text looks weird. This CL removes the unnecessary z-index.

Bug: b/172879638
Test: Manually toggle barcode scanning and open settings menu.
Change-Id: I5077ed3a89eb0421d07d413314c43049fd7db10c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2604743
Commit-Queue: Shik Chen <shik@chromium.org>
Reviewed-by: Wei Lee <wtlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#839433}

chromeos/components/camera_app_ui/resources/css/main.css

@@ -1563,7 +1563,6 @@ body:not(.mode-switching):not(.streaming):not(.review-result) #spinner {
   padding-top: calc(100% / 3);
   visibility: hidden;
   width: calc(100% / 3);
-  z-index: 30;
 }
 
 body.photo.scan-barcode .barcode-scan-box {