[FEA] Remove UCX code in favor UCXX implementation

[pentschev]

#1983 introduced UCXX support in both C++ and Cython/Python. Up until now we had UCX-Py implementation using the original UCX implementation, while the Python UCXX implementation used the C++ UCXX implementation, this was being selected via the is_ucxx argument. With the UCX-Py removal in #2791 due to the upcoming archival of UCX-Py the original pure-UCX code will not be used nor tested anymore and should be removed. The C++ UCXX implementation is also shorter and thus hopefully easier to maintain, so that is an advantage to the existing C++ codebase as well.

[vyasr]

When this task is completed, we should also stop linking to the ucx::ucp target in raft's CMake code, and once that is done we should revert rapidsai/ucxx#507. That ucxx PR was a patch that in hindsight is not strictly correct. It brings the ucx target into scope for consumers of ucxx, but really that shouldn't be necessary. Either:

1. A package only depends on ucx via ucxx, in which case 507 is completely superfluous, or
2. A package uses both ucx and ucxx directly, in which case they should find ucx again themselves. I kind of rushed in Make ucx target global ucxx#507 to unblock things, but the reason it was necessary is because Remove CMake find UCX package #2798 wasn't really right and shouldn't work. If raft is currently using ucx directly it should find ucx on its own. Make ucx target global ucxx#507 exported the target globally so that raft didn't have to.

tl;dr before closing this issue we should also revert rapidsai/ucxx#507 and make sure that everything is working as expected.