px4_middleware: remove that header and move px4::init to px4_init.h

delete include:
for i in $(grep -rl 'px4_middleware.h' src platforms); do sed -i '/#include <px4_middleware.h/d' $i; done

platforms/common/work_queue/wqueue_test/wqueue_main.cpp

@@ -40,7 +40,6 @@
 #include "wqueue_test.h"
 
 #include <px4_log.h>
-#include <px4_middleware.h>
 #include <px4_app.h>
 #include <stdio.h>