Revert "cache shourtcut image"

This reverts commit 050f9b12.

Reason for revert: Test failure on ub-launcher3-master (b/140998509)

Possible Root Cause: This happens when fallback image or default icon was
used for shortcut icon, but shortcut caching logic do not have the logic
to create either fallback image or default icon. So upon updating icon cache,
the icon remains to be null, causing an NPE.

Change-Id: I1ee3bb7a3cab2af5730c2ee77d9370c1578a9ad6