[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.*}

首先阅读 ArrowBuf 及其对 ByteBuf 的继承关系，以确定 issue 中描述的读写索引功能。在决定范围之前，跟踪相关 API 的使用情况；完成的标准是 ArrowBuf 不再暴露该索引功能，同时其余缓冲区行为仍能正常工作。

由索引模型根据 Issue 内容生成。