[4/N] Remove unused code paths in each OomAdjuster impl

Primarily trim out all the dead code for handling the Legacy traversal
from OomAdjusterModernImpl.

Flag: EXEMPT pure refactor
Test: atest MockingOomAdjusterTests
Test: builds and flashes
Bug: 306731120

Change-Id: Idfd77557c03a7ba8971a2bb55c11b482d4daa3fe