Remove legacy betweenness centrality (rapidsai#3829)

The python API is now leveraging the CAPI for both [betweenness](rapidsai#2971) and [edge betweenness centrality](rapidsai#3672) therefore, the legacy code is no longer used anywhere. This PR cleanup the C++ API.

closes rapidsai#2651 
closes rapidsai#3272

Authors:
  - Joseph Nke (https://github.com/jnke2016)
  - Chuck Hastings (https://github.com/ChuckHastings)

Approvers:
  - Chuck Hastings (https://github.com/ChuckHastings)

URL: rapidsai#3829

cpp/CMakeLists.txt

@@ -195,7 +195,6 @@ set(CUGRAPH_SOURCES
     src/utilities/path_retrieval.cu
     src/structure/legacy/graph.cu
     src/linear_assignment/legacy/hungarian.cu
-    src/traversal/legacy/bfs.cu
     src/link_prediction/legacy/jaccard.cu
     src/link_prediction/legacy/overlap.cu
     src/link_prediction/jaccard_sg.cu
@@ -234,7 +233,6 @@ set(CUGRAPH_SOURCES
     src/cores/k_core_sg.cu
     src/cores/k_core_mg.cu
     src/components/legacy/connectivity.cu
-    src/centrality/legacy/betweenness_centrality.cu
     src/generators/generate_rmat_edgelist.cu
     src/generators/generate_bipartite_rmat_edgelist.cu
     src/generators/generator_tools.cu