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);
   }