Revert "Auto-close if you swipe away the last notification."

This reverts commit 8cc36e572876f3bb9197f0109db3042466e5f917.

It didn't work as advertised anyway: It only worked if there are no
persistent notifications, and it also closes the notification area if
the notification is closed by the app itself.

Change-Id: I16f0fb6cf41d20c3a3bd4278446edab780c1fc77