Only cancel CoroutineScope of TaskThumbnailView instead of removing from RV.

Removing TVs from RV was causing failures of CUJs (only in tests) when enabling TTV flag.

Bug: 390156722
Flag: com.android.launcher3.enable_refactor_task_thumbnail
Test: TaplTestsQuickstep.testOverview
Test: TaplTestsKeyboardQuickSwitch.testLaunchOverviewTask_fromHomeAllApps
Change-Id: I0aae99891941c04b543d1e29db9d6d87bc89ed8e