cpp: remove @return doc for void functions

For functions returning nothing, there is no need to document
with @return, as Doxgen complains about "documented empty
return type of ...".

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

samples/subsys/cpp/cpp_synchronization/src/main.cpp

@@ -98,12 +98,9 @@ int cpp_semaphore::wait(int timeout)
 }
 
 /**
- *
  * @brief Signal a semaphore
  *
  * This routine signals the specified semaphore.
- *
- * @return N/A
  */
 void cpp_semaphore::give(void)
 {

subsys/cpp/cpp_dtors.c

@@ -24,7 +24,7 @@ __weak void *__dso_handle;
  * Function does nothing at the moment, assuming the global objects
  * do not need to be deleted
  *
- * @return N/A
+ * @retval 0 on success.
  */
 int __cxa_atexit(void (*destructor)(void *), void *objptr, void *dso)
 {

subsys/cpp/cpp_init_array.c

@@ -16,8 +16,6 @@ extern func_ptr __init_array_end[];
 
 /**
  * @brief Execute initialization routines referenced in .init_array section
- *
- * @return N/A
  */
 void __do_init_array_aux(void)
 {

subsys/cpp/cpp_virtual.c

@@ -14,8 +14,6 @@
  *
  * This routine is needed for linking C++ code that uses pure virtual
  * functions.
- *
- * @return N/A
  */
 void __cxa_pure_virtual(void)
 {