[GPU-Plugin] Major refactor (#2644)

* Removal of redundant code/files.
* Removal of exact namespace in GPU plugin
* Revert double precision histograms to single precision for performance on Maxwell/Kepler

src/tree/tree_updater.cc

@@ -32,5 +32,9 @@ DMLC_REGISTRY_LINK_TAG(updater_prune);
 DMLC_REGISTRY_LINK_TAG(updater_fast_hist);
 DMLC_REGISTRY_LINK_TAG(updater_histmaker);
 DMLC_REGISTRY_LINK_TAG(updater_sync);
+#ifdef XGBOOST_USE_CUDA
+DMLC_REGISTRY_LINK_TAG(updater_gpu);
+DMLC_REGISTRY_LINK_TAG(updater_gpu_hist);
+#endif
 }  // namespace tree
 }  // namespace xgboost