replace const_iterator to iterator when erasing

[Halcao]

The code cannot compile due to error:

/src/LoopClosing.cc:2418:51: error: no matching function for call to 'std::list<ORB_SLAM3::KeyFrame*>::erase(std::list<ORB_SLAM3::KeyFrame*>::const_iterator&)'
                 it = mlpLoopKeyFrameQueue.erase(it);

I looked it up on stackoverflow, and it says that const_iterator indicates a position and cannot be the argument of erase until c++11 had overload erase for const_iterator. calling-erase-with-iterator-vs-const-iterator

The question is that I am using g++ of gcc version 4.8.5 20150623 (Red Hat 4.8.5-36) (GCC) which supports c++ 11. So for the compatibility of c++ versions concern, we'd better replace const_iterator to iterator, which works on my computer.