Remove robin-hood-hashing dependency

[imwints]

As per #617 using a specialized hash map/set is not worth the effort

[aristocratos]

This one is gonna have to wait a little bit because of the amount of touched code.

But in the meantime, would you be willing to do some performance testing between std::unordered_map and the robin hood hash maps?

It would be interesting to see if we actually are giving up any performance with the switch.

[aristocratos]

Btw @nobounce, I hope you know that I really appreciate the interest you've taken in btop and your efforts to improve the code!
I sadly don't have as much free time as I used to.
But you and the others that graciously choose to contribute improvements and new functionality to btop is a great help! ❤

[imwints]

It would be interesting to see if we actually are giving up any performance with the switch.

I had a look at it. I only looked at the memory footprint of btop itself. unordered_dense is around 400kB better than the current version with robin-hood-hashing. The standard implementation (of libstdc++) uses around 2M more RAM than the current implementation. In both this and the previous PR I deleted calls to compact() without a replacement. The proper function to replace it is rehash() in both the standard lib and unordered_dense. This saves some memory as well