[posix] remove unused node id (#5416)

src/posix/platform/platform-posix.h

@@ -97,12 +97,6 @@ struct RadioProcessContext
     const fd_set *mWriteFdSet;
 };
 
-/**
- * Unique node ID.
- *
- */
-extern uint64_t gNodeId;
-
 /**
  * This function initializes the alarm service used by OpenThread.
  *

src/posix/platform/system.cpp

@@ -46,8 +46,6 @@
 
 #include "common/code_utils.hpp"
 
-uint64_t gNodeId = 0;
-
 otInstance *otSysInit(otPlatformConfig *aPlatformConfig)
 {
     otInstance *        instance = nullptr;