Bug#36158022: Replace the Robin Hood library [patch 4, unordered_dense]

Remove the robin-hood-hashing library and replace it with
ankerl::unordered_dense version 4.4.0 from
https://github.com/martinus/unordered_dense.

It was more or less a drop-in replacement. What needed some adjustment
was that the new library didn't have the same helper function for
estimating the memory used by the hash map. Manually calculated the
size of the bucket vector and the value vector instead.

Change-Id: I24745ab716be3c8607c877b24773cc7c30970402
(cherry picked from commit 8bf89a6bb2d88e1b474ba324243c429ab3646f46)

Author: kahatlen