Remove unused 3rdparty dependencies.
Wire up tests not linked to src/test/python:all
Remove unused tests.

Testing Done:
Listed leaf dependencies:

```bash
for target in $(./pants goal list :: 2>/dev/null); do
  if [[ -z "$(./pants goal dependees $target 2>/dev/null)" ]]; then
    echo $target
  fi
done
```

Reviewed at https://reviews.apache.org/r/29267/