Removing deprecated UPSTREAM/DOWNSTREAM symbols

Now 2.0.8 has been released, I don't see any real need to keep them
there.

Revert "Adding support for zmq 2.0.7 with a hacky preprocessor check"

This reverts commit 9d38da847d.

zmq.c

@@ -357,14 +357,8 @@ LUALIB_API int luaopen_zmq(lua_State *L)
     set_zmq_const(REP);
     set_zmq_const(XREQ);
     set_zmq_const(XREP);
-
-#if defined(ZMQ_PULL) && defined(ZMQ_PUSH)
     set_zmq_const(PULL);
     set_zmq_const(PUSH);
-#else
-    set_zmq_const(UPSTREAM);
-    set_zmq_const(DOWNSTREAM);
-#endif
 
     set_zmq_const(HWM);
     set_zmq_const(SWAP);