Cleanup the IPCLIENT_MULTICAST_NS_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: 315056647
Test: atest NetworkStackIntegrationTests
Change-Id: I6305897cd307ebc2d65f6d623a44ec33c8abd5cc
3 files changed