Remove dead code

Removed dead code that is no longer used due to migration to the drop
target manager.

Bug: 411505605
Flag: com.android.wm.shell.enable_bubble_bar
Test: Manual. Dragged application icon from the app grid shown inside
the application.

Change-Id: I2dcf51593d4e41d53f5f20413fd5b2f10d2dff8e