Remove use of `READ_COMMAND` in znode.g

[fingolfin]

I think you can simply replace it by READ_COMMAND_REAL -- after all, these two functions only differ in their return values, and the zmq code ignores the return value.

[markuspf]

Resovled in dc5e7a985a4dc40b79734b5350cdb711413c0839 (and released in 0.7)