Delete codestyle-intellj.xml and update codestyle-eclipse.xml, then enable eclipse formatter in spotless maven plugin

https://github.com/apache/incubator-pinot/pull/6782 add two basic auto formatting rules (auto import order and remove unused imports). We should apply more strict rules so that `mvn spotless:apply` gives same result as the IDE auto-format.

To achieve that, we need:

- codestyle-intellj.xml file can be deleted now. Intellj support import eclipse code formatter settings since 2014. Maintaining one file is much easier that maintaining two. Corresponding dev guide doc should be updated also.
- the existing codestyle-eclipse.xml is not compatible with the spotless maven plugin. We need update it.
- After codestyle-eclipse.xml is updated to version 15. We can enable eclipse code formatter in spotless maven plugin.

Start with codestyle-intellj.xml, codestyle-eclipse.xml, the Spotless Maven plugin configuration, and the corresponding dev guide. Run mvn spotless:apply to compare the formatter result with the IDE settings. Done means the IntelliJ file is removed, the Eclipse formatter is compatible with Spotless at version 15, the plugin uses it, and the guide reflects the single-file setup.

Written by the indexing model from the issue text.