Cleanup RABIT. (#6290)

* Remove recovery and MPI speed tests.
* Remove readme.
* Remove Python binding.
* Add checks in C API.

src/common/random.h

@@ -143,7 +143,7 @@ class ColumnSampler {
   */
   ColumnSampler() {
     uint32_t seed = common::GlobalRandom()();
-    rabit::Broadcast(&seed, sizeof(seed), 0, "seed");
+    rabit::Broadcast(&seed, sizeof(seed), 0);
     rng_.seed(seed);
   }
 

src/data/data.cc

@@ -714,8 +714,7 @@ DMatrix* DMatrix::Load(const std::string& uri,
   /* sync up number of features after matrix loaded.
    * partitioned data will fail the train/val validation check
    * since partitioned data not knowing the real number of features. */
-  rabit::Allreduce<rabit::op::Max>(&dmat->Info().num_col_, 1, nullptr,
-    nullptr, fname.c_str());
+  rabit::Allreduce<rabit::op::Max>(&dmat->Info().num_col_, 1);
   // backward compatiblity code.
   if (!load_row_split) {
     MetaInfo& info = dmat->Info();

src/learner.cc

@@ -546,8 +546,7 @@ class LearnerConfiguration : public Learner {
         num_feature = std::max(num_feature, static_cast<uint32_t>(num_col));
       }
 
-      rabit::Allreduce<rabit::op::Max>(&num_feature, 1, nullptr, nullptr,
-                                       "num_feature");
+      rabit::Allreduce<rabit::op::Max>(&num_feature, 1);
       if (num_feature > mparam_.num_feature) {
         mparam_.num_feature = num_feature;
       }