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

src/examples/hello/hello_main.cpp

@@ -40,8 +40,8 @@
 
 #include "hello_example.h"
 
-#include <px4_middleware.h>
 #include <px4_app.h>
+#include <px4_init.h>
 #include <stdio.h>
 
 int PX4_MAIN(int argc, char **argv)