[SB] Finish cleaning up icon callbacks in SBIconController.

1) Deleted `removeAllIconsForSlot` which is actually unused.
2) I decided to not rename `setIcon` to `setInternalIcon` because it's
   used in so many places, so I just removed the TODO.

Fixes: 265307726
Test: atest StatusBarIconControllerImplTest
Change-Id: I88c8225e7bde05f1946ba9e7e819e2b76114204b