event: remove CONFIG_EVENT_DYNAMIC check in event_register()

The whole event_register() function is wrapped in EVENT_DYNAMIC #ifdef
checks, so the inner check is not needed:

 #if CONFIG_IS_ENABLED(EVENT_DYNAMIC)
 ...
 int event_register(...)
 {
     ...
     if (!CONFIG_IS_ENABLED(EVENT_DYNAMIC))
         return -ENOSYS;
 }
 #endif

Signed-off-by: Ovidiu Panait <ovpanait@gmail.com>