[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.*}

Comece lendo ArrowBuf e sua herança de ByteBuf para identificar a funcionalidade de índice de leitura e escrita descrita na issue. Rastreie o uso da API relacionada antes de decidir o escopo; considera-se concluído quando ArrowBuf não expõe mais essa funcionalidade de índice, enquanto o comportamento restante do buffer continua funcionando.

Escrita pelo modelo de indexação a partir do texto da issue.