[Misc] Remove the cora example dataset, too big. (#6547)

Co-authored-by: Ubuntu <ubuntu@ip-172-31-28-63.ap-northeast-1.compute.internal>

examples/sampling/graphbolt/quickstart/cora/metadata.yaml

-dataset_name: cora
-feature_data:
-- domain: node
-  format: numpy
-  in_memory: true
-  name: feat
-  path: node-feat.npy
-  type: null
-graph:
-  nodes:
-    - num: 2708
-  edges:
-    - path: edges.csv
-tasks:
-- name: node_classification
-  num_classes: 7
-  train_set:
-  - data:
-    - format: numpy
-      in_memory: true
-      name: seed_nodes
-      path: nodeclassification/train-node.npy
-    - format: numpy
-      in_memory: true
-      name: labels
-      path: nodeclassification/train-label.npy
-    type_name: null
-  validation_set:
-  - data:
-    - format: numpy
-      in_memory: true
-      name: seed_nodes
-      path: nodeclassification/valid-node.npy
-    - format: numpy
-      in_memory: true
-      name: labels
-      path: nodeclassification/valid-label.npy
-    type_name: null
-  test_set:
-  - data:
-    - format: numpy
-      in_memory: true
-      name: seed_nodes
-      path: nodeclassification/test-node.npy
-    - format: numpy
-      in_memory: true
-      name: labels
-      path: nodeclassification/test-label.npy
-    type_name: null
-- name: link_prediction
-  train_set:
-  - data:
-    - format: numpy
-      in_memory: true
-      name: node_pairs
-      path: linkprediction/train-node-pairs.npy
-    type_name: null
-  test_set:
-  - data:
-    - format: numpy
-      in_memory: true
-      name: node_pairs
-      path: linkprediction/test-node-pairs.npy
-    - format: numpy
-      in_memory: true
-      name: negative_srcs
-      path: linkprediction/test-negative-srcs.npy
-    - format: numpy
-      in_memory: true
-      name: negative_dsts
-      path: linkprediction/test-negative-dsts.npy
-    type_name: null