C++20 future improvement

[pvmm]

I was going to suggest to replace this loop with a call to std::erase_if(m, [](auto& entry) { return entry->index == -1; }); But this doesn't work (yet) because:

• std::erase_if() is only available in c++20
• it works on STL containers, but not on QMap. Any reason why you've chosen QMap instead of std::map?

So no need to change anything (yet).

Originally posted by @m9710797 in https://github.com/openMSX/debugger/pull/104#discussion_r838614870