AP_HAL_Linux: 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_Linux/Semaphores.cpp

@@ -8,12 +8,6 @@ using namespace Linux;
 
 // 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_Linux/Semaphores.h

@@ -18,9 +18,4 @@ protected:
     pthread_mutex_t _lock;
 };
 
-class Semaphore_Recursive : public Semaphore {
-public:
-    Semaphore_Recursive();
-};
-    
 }