Remove dead code from NotifEntryManager

All removed code is unused; since all changes are relegated to
NotificationEntryManager.java, if this CL still builds, it is proof that
the code in question was not referenced anywhere (assuming no
reflection).

Bug: 200269355
Test: atest SystemUITests
Change-Id: If24a9a92c6af5c2edd4fe1b628bb720a6a12a311
Merged-In: If24a9a92c6af5c2edd4fe1b628bb720a6a12a311