Fix: Wikipedia - save memory by replacing root.clear with elem.clear
see: https://github.com/huggingface/datasets/issues/2031

What I did:
- replace root.clear with elem.clear
- remove lines to get root element
- $ make style
- $ make test
  - some tests required some pip packages, I installed them.

test results on origin/master and my branch are same. I think it's not related on my modification, isn't it?
```
==================================================================================== short test summary info ====================================================================================
FAILED tests/test_arrow_writer.py::TypedSequenceTest::test_catch_overflow - AssertionError: OverflowError not raised
============================================================= 1 failed, 2332 passed, 5138 skipped, 70 warnings in 91.75s (0:01:31) ==============================================================
make: *** [Makefile:19: test] Error 1

```

Is there anything else I should do?
The error you got is minor and appeared in the last version of pyarrow, we'll fix the CI to take this into account. You can ignore it