BUG-650: improve toYangInstanceIdentifier() speed

This removes a superfluous call to getCodecContextNode(), since that
call is already performed by the InstanceIdentifierCodec.

Change-Id: I2077e70517adf6795f805b6c37c0a7e29b4fb9e5
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-650: improve toYangInstanceIdentifier() speed"