Fix compiler warnings.

[trivialfis]

• Remove/fix unused parameters
• Remove deprecated code in rabit.
• Update dmlc-core.

Using nvcc 11.6 and gcc 9.4.0, only 1 warning remains:

hist_util.cc:43:89: warning: ‘void* memset(void*, int, size_t)’ clearing an object of non-trivial type ‘class xgboost::detail::GradientPairInternal<double>’; use assignment or value-initialization instead [-Wclass-memaccess]
   43 |   memset(hist.data() + begin, '\0', (end - begin) * sizeof(xgboost::GradientPairPrecise));
      |                                                                                         ^

This one requires some benchmarking. We can prepare for turning warnings into errors on CI once we start using a newer version of CUDA.

[trivialfis]

https://xgboost-ci.net/blue/organizations/jenkins/xgboost/detail/PR-8022/4/pipeline

[RAMitchell]

!!!!???