delete activation-key function and tests

Changes Made are:
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- Function to delete selected key
- three tests:
   `test_negative_delete_activation_key_1`
  ` test_positive_delete_activation_key_1`
   `test_positive_delete_activation_key_2`
- `wait_for_ajax` is not working with activation-key, I tried to set the timeout to '120' as well. Finally, I replaced it again with `sleep_for_seconds`.