Cleanup the IPCLIENT_ACCEPT_IPV6_LINK_LOCAL_DNS_VERSION flag.

This fix (accept the IPv6 link-local DNS server) has been enabled by
default and rolled out via M-2023-08 train, cleanup the constant flag
string and associated dead code in the IpClientLinkObserver and tests.

Bug: 300003598
Test: atest NetworkStackRootTests
Change-Id: I63e295a3b3a60189039b62797c9b3058f6ca520f
3 files changed