[port][signal] Remove obsolete PORT_READABLE signal bit

Ports cannot be waited on via object wait, only via port wait. This
signal bit makes no sense and is unused.

Change-Id: I74452eda2d1280584af86f2d32e2402154a2e177
diff --git a/system/public/zircon/types.h b/system/public/zircon/types.h
index eacb4f2..72704bd 100644
--- a/system/public/zircon/types.h
+++ b/system/public/zircon/types.h
@@ -121,9 +121,6 @@
 #define ZX_SOCKET_ACCEPT            __ZX_OBJECT_SIGNAL_8
 #define ZX_SOCKET_SHARE             __ZX_OBJECT_SIGNAL_9
 
-// Port
-#define ZX_PORT_READABLE            __ZX_OBJECT_READABLE
-
 // Fifo
 #define ZX_FIFO_READABLE            __ZX_OBJECT_READABLE
 #define ZX_FIFO_WRITABLE            __ZX_OBJECT_WRITABLE