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/drivers/qshell/qurt/qshell.cpp

@@ -43,7 +43,6 @@
 #include <px4_log.h>
 #include <px4_time.h>
 #include <px4_posix.h>
-#include <px4_middleware.h>
 #include <px4_defines.h>
 #include <dspal_platform.h>
 

src/drivers/qshell/qurt/qshell_main.cpp

@@ -38,9 +38,9 @@
  * @author Nicolas de Palezieux <ndepal@gmail.com>
  */
 
-#include <px4_middleware.h>
 #include <px4_log.h>
 #include <px4_app.h>
+#include <px4_init.h>
 #include "qshell.h"
 
 extern "C" __EXPORT int qshell_entry(int argc, char **argv);