Cleanup the IP_REACHABILITY_MCAST_RESOLICIT_VERSION experiment flag.

The feature has been enabled by default and rolled out via M-2023-07
train, cleanup the constant flag string and associated dead code.

Bug: 314216940
Test: atest NetworkStackIntegrationTests
Test: atest NetworkStackTests
Change-Id: Ic6744660f23a29e8867125ac504f6f201ff06969
4 files changed