Remove concurrent_unordered_multimap. (#10642)

The `concurrent_unordered_multimap` is no longer used in libcudf. It has been replaced by `cuco::static_multimap`. The majority of the refactoring was done in PRs #8934 and #9704. A similar effort is in progress for `concurrent_unordered_map` and `cuco::static_map` in #9666 (and may depend on porting some optimizations from libcudf to cuco -- need to look into this before doing a direct replacement).

This partially resolves issue #10401.

cc: @PointKernel @vyasr

Authors:
  - Bradley Dice (https://github.com/bdice)

Approvers:
  - Yunsong Wang (https://github.com/PointKernel)
  - Vyas Ramasubramani (https://github.com/vyasr)
  - Jake Hemstad (https://github.com/jrhemstad)

URL: #10642