Revert "Port alltoall[v]" (#325)

This reverts commit f4d5d3d620.

test/test_Gather.cpp

@@ -66,6 +66,6 @@ namespace CorrectnessTests
                                 testing::Values(2,3,4,5,6,7,8),
                                 // In-place or not
                                 testing::Values(false),
-                                testing::Values("RCCL_ALLTOALL_KERNEL_DISABLE=1")),
+                                testing::Values("")),
                             CorrectnessTest::PrintToStringParamName());
 } // namespace