Cleanup some old FIXMEs

[aprokop]

No description provided.

[aprokop]

Removal of the DBSCAN verification fix still results in

<snip>/benchmarks/cluster/ArborX_DBSCANVerification.hpp:483:17: error: expected primary-expression before 'index'
  483 |   ArborX::DistributedTree index(comm, exec_space,
      |                 ^~~~~

issue with CUDA 12.0.1. We can try to upgrade our CUDA requirement to 12.2 and retry, or return the workaround. I think I prefer the former to align better with Kokkos 5.0.

[aprokop]

OK, the distributed tree deduction guide does not work even with CUDA 12.8. So, dropping that patch.

[aprokop]

Everything passes, some irrelevant warnings from container build.