[Java] Remove reader index and writer index from ArrowBuf

Reader and writer index and functionality doesn't belong on a chunk of memory and is due to inheritance from ByteBuf. As part of removing ByteBuf inheritance, we should also remove reader and writer indexes from ArrowBuf functionality. It wastes heap memory for rare utility. In general, a slice can be used instead of a reader/writer index pattern.

**Reporter**: [Jacques Nadeau](https://issues.apache.org/jira/browse/ARROW-7494) / @jacques-n
#### PRs and other links:
- [GitHub Pull Request apache/arrow#6133](https://github.com/apache/arrow/pull/6133)

_{**Note**: *This issue was originally created as [ARROW-7494](https://issues.apache.org/jira/browse/ARROW-7494). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

首先閱讀 ArrowBuf 及其對 ByteBuf 的繼承關係，以確定 issue 中描述的讀寫索引功能。在決定範圍之前，追蹤相關 API 的使用情況；完成的標準是 ArrowBuf 不再暴露該索引功能，同時其餘緩衝區行為仍能正常運作。

由索引模型根據 Issue 內容生成。