AP_HAL_SITL: make all semaphores recursive

the cost is very similar and this prevents an easy coding error which
can occur on less used code paths

libraries/AP_HAL_SITL/AP_HAL_SITL_Namespace.h

@@ -12,7 +12,6 @@ class ADCSource;
 class RCInput;
 class Util;
 class Semaphore;
-class Semaphore_Recursive;
 class GPIO;
 class DigitalSource;
 class HALSITLCAN;

libraries/AP_HAL_SITL/Semaphores.cpp

@@ -11,12 +11,6 @@ using namespace HALSITL;
 
 // construct a semaphore
 Semaphore::Semaphore()
-{
-    pthread_mutex_init(&_lock, nullptr);
-}
-
-// construct a recursive semaphore (allows a thread to take it more than once)
-Semaphore_Recursive::Semaphore_Recursive()
 {
     pthread_mutexattr_t attr;
     pthread_mutexattr_init(&attr);

libraries/AP_HAL_SITL/Semaphores.h

@@ -16,11 +16,3 @@ public:
 protected:
     pthread_mutex_t _lock;
 };
-
-
-class HALSITL::Semaphore_Recursive : public HALSITL::Semaphore {
-public:
-    Semaphore_Recursive();
-};
-
-