Remove unneeded pair-iterator benchmark

[davidwendt]

Description

Removes the pair-iterator benchmark logic. The remaining benchmarks use the null-replacement-iterator which uses the libcudf pair-iterator internally. There is no need for benchmarking this unique iterator pattern that is not used by libcudf.

The cpp/benchmarks/iterator/iterator.cu failed to compile with gcc 12 because the sum-reduce function cannot resolve adding thrust::pair objects together likely due to some recent changes in CCCL. Regardless, adding thrust::pair objects is not something we need to benchmark. The existing benchmark benchmarks libcudf's usage of the internal pair-iterator correctly.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[PointKernel]

Nice cleanup

[davidwendt]

/merge