Revert "Check that ViewTreeObserver is alive before unregistering listener."

This reverts commit b274079f.

Reason for revert: Suspected as cause of app startup performance regression. Running the performance tests locally is taking more time than expected, so I'm reverting while I'm waiting.

Bug: 117611397
Change-Id: I2f78bbc68cdc5e5a19b740a986396d3cad4e574c