Cleanup IP_REACHABILITY_IGNORE_NEVER_REACHABLE_NEIGHBOR_VERSION flag.

This feature has been enabled by default and roll out via M-2024-11
mainline train, cleanup the experiment flag in the production code.

Bug: 382194967
Test: atest NetworkStackIntegrationTests
Test: atest NetworkStackRootTests
Test: atest NetworkStackTests
Change-Id: I4710283b3ccf22486b69a48e1253c06a2d593280
4 files changed