Change `Mutex::unlock()` function definition - void return value

This is done for consistency with the new version of `Mutex::lock()` member function which does not return status.

rtos/Mutex.cpp

@@ -108,10 +108,13 @@ bool Mutex::trylock_until(uint64_t millisec)
     }
 }
 
-osStatus Mutex::unlock()
+void Mutex::unlock()
 {
     _count--;
-    return osMutexRelease(_id);
+
+    osStatus status = osMutexRelease(_id);
+
+    MBED_ASSERT(status == osOK);
 }
 
 osThreadId Mutex::get_owner()

rtos/Mutex.h

@@ -135,16 +135,12 @@ public:
      */
     bool trylock_until(uint64_t millisec);
 
-    /** Unlock the mutex that has previously been locked by the same thread
-      @return status code that indicates the execution status of the function:
-              @a osOK the mutex has been released.
-              @a osErrorParameter internal error.
-              @a osErrorResource the mutex was not locked or the current thread wasn't the owner.
-              @a osErrorISR this function cannot be called from the interrupt service routine.
+    /**
+      Unlock the mutex that has previously been locked by the same thread
 
       @note You cannot call this function from ISR context.
      */
-    osStatus unlock();
+    void unlock();
 
     /** Get the owner the this mutex
       @return  the current owner of this mutex.