Remove workaround of delaying remove task

The timing doesn't work on the latest code base. The onStop
of activity still almost has no chance to be called after
swiping the task to kill process.

And the friendly activity lifecycle will be controlled by
removeTask since Id69a8ba77da16a13f7a3413bb004fc2206f4b4e8.

Bug: 184899234
Bug: 213583589
Test: No behavior change after swiping task out from recents
Change-Id: If4110069834964571df7128b119fc6c782a03b34