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