[ja] `.filter` is used instead of `.map` for non-filter methods

On https://github.com/EleutherAI/dps/blob/bec4078f341037879feab1d5c82668745b28aa55/dps/spark/jobs/japanese_job.py#L64-L75 there are several cases where we are using `.filter` but instead it should be a `.map`.

For example https://github.com/EleutherAI/dps/blob/bec4078f341037879feab1d5c82668745b28aa55/dps/spark/jobs/japanese_job.py#L73
calls
https://github.com/EleutherAI/dps/blob/bec4078f341037879feab1d5c82668745b28aa55/dps/spark/prep/japanese_prep.py#L64-L67
but in effect this is doing nothing because the expression within .filter is always is true, as long as text is non-empty:

```python
>>> def reduce_japanese_emoticon(text):
... text = re.sub("w{3,}", "www", text)
... text = re.sub("笑{2,}", "笑", text)
... return text
>>> rdd = sc.parallelize([{'text': 'wwwwasdf'}, {'text': '1234笑笑笑'}, {'text': ''}])
>>> rdd.filter(lambda x: reduce_japanese_emoticon(x['text'])).collect()
[{'text': 'wwwwasdf'}, {'text': '1234笑笑笑'}]
```

Thus, I think the following cases of `.filter` are simply doing nothing instead of the intended preprocessing:
- `preprocess_text` on https://github.com/EleutherAI/dps/blob/bec4078f341037879feab1d5c82668745b28aa55/dps/spark/jobs/japanese_job.py#L70
- `reduce_japanese_emoticon` on https://github.com/EleutherAI/dps/blob/bec4078f341037879feab1d5c82668745b28aa55/dps/spark/jobs/japanese_job.py#L73
- `remove_symbols` on https://github.com/EleutherAI/dps/blob/bec4078f341037879feab1d5c82668745b28aa55/dps/spark/jobs/japanese_job.py#L75

The remaining calls to methods that end with `_filter` (e.g. `japanese_bad_words_filter`, `doc_len_filter`, etc.) are actually filter methods that return booleans so they should be OK.