rtems: Remove deprecated rtems_extension

This type was deprecated in RTEMS 5.1.

Close #3988.

cpukit/include/rtems/extension.h

@@ -26,9 +26,6 @@
 extern "C" {
 #endif
 
-typedef User_extensions_routine
-  rtems_extension RTEMS_DEPRECATED;
-
 /**
  * @defgroup ClassicUserExtensions User Extensions
  *

cpukit/include/rtems/score/userext.h

@@ -26,8 +26,6 @@ extern "C" {
 
 struct _Thread_Control;
 
-typedef void User_extensions_routine RTEMS_DEPRECATED;
-
 /**
  * @defgroup RTEMSScoreUserExt User Extension Handler
  *