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/nuttx/src/px4/common/px4_nuttx_impl.cpp

@@ -38,7 +38,6 @@
  */
 
 #include <px4_defines.h>
-#include <px4_middleware.h>
 #include <drivers/drv_hrt.h>
 
 namespace px4
@@ -49,9 +48,4 @@ void init(int argc, char *argv[], const char *process_name)
 	printf("process: %s\n", process_name);
 }
 
-uint64_t get_time_micros()
-{
-	return hrt_absolute_time();
-}
-
 }