Cleanup unused region sampling code for Bubbles.

This is not longer needed since the color of the handle is now set by
BubbleBarCaptionView, which is determined based on the background color
of the Task.

Fix: 403612933
Test: wm presubmit
Flag: com.android.wm.shell.enable_create_any_bubble
Change-Id: I49e6c3776b75f68dcc1c38ab6f6168c7f9e9e945