Remove redundant a11y announcement upon removal of workspace item.

When a workspace item is removed, launcher currently shows a Snackbar which contains a message saying "item removed". This renders the a11y announcement in DeleteDropTarget#onAccessibilityDrop redundant since it's saying exactly the same string.


Bug: 378032433
Change-Id: Iaf3b42404ecd382b1ee15f670cc4ebe0124218d2
Test: manual
Flag: EXEMPT bugfix