Fix dataset builder default version
_The documentation is not available anymore as the PR was closed or merged._ This PR requires one of these other PRs being merged first:
- #4359 
- huggingface/doc-builder#211
Currently, when using a custom config (subclass of `BuilderConfig`), default version set at the builder level is ignored: we must set default version in the custom config class.

However, when loading a dataset with `config_kwargs` (for a configuration not present in `BUILDER_CONFIGS`), the default version set in the custom config is ignored and "0.0.0" is used instead:
```python
ds = load_dataset("wikipedia", language="co", date="20220501", beam_runner="DirectRunner")
```
generates the following config:
```python
WikipediaConfig(name='20220501.co', version=0.0.0, data_dir=None, data_files=None, description='Wikipedia dataset for co, parsed from 20220501 dump.')
```
with version "0.0.0" instead of "2.0.0".

See as a counter-example, when the config is present in `BUILDER_CONFIGS`:
```python
ds = load_dataset("wikipedia", "20220301.fr", beam_runner="DirectRunner")
```
generates the following config:
```python
WikipediaConfig(name='20220301.fr', version=2.0.0, data_dir=None, data_files=None, description='Wikipedia dataset for fr, parsed from 20220301 dump.')
```
with correct version "2.0.0", as set in the custom config class.


The reason for this is that `DatasetBuilder` has a default VERSION ("0.0.0") that overwrites the default version set at the custom config class.

This PR:
- Removes the default VERSION at `DatasetBuilder` (set to None, so that the class attribute exists but it does not override the custom config default version).
- Note that the `BuilderConfig` class already sets a default version = "0.0.0"; no need to pass this from the builder.