Eliminate KeyedListNodeCodecContext.Unordered.getKey()

Eliminate KeyedListNodeCodecContext.Unordered.getKey()

This method and the methodhandle invoker are completely superfluous,
as we can directly access key() from the created object. While capturing
a MethodHandle lowers what method can be invoked, it does not really
buy us much, as the observed class will we always the same.

That property is visible enough (wired through an invokeExact() on the
constructor), hence JIT should be able to optimize without this help.

Change-Id: Id24eac8e74782f750576c90d60485b2dcbdcff9e
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>