Remove dependencies on httplib2 and google-auth-httplib2

This library still has dependencies on `httplib2` and `google-auth-httplib2`. Most of the other Google Python libs have [migrated away](https://github.com/googleapis/google-cloud-python/issues/1346) from these. Specifically, `httplib2` has [known thread-safety problems](https://github.com/googleapis/google-api-python-client/blob/master/docs/thread_safety.md), and `google-auth-httplib2` seems to have been developed as a backward compat package. Are there any plans for migrating these dependencies out?

Revise primeiro a discussão de migração vinculada do google-cloud-python e a thread da issue. A meta de conclusão declarada é remover as dependências httplib2 e google-auth-httplib2, mas nenhum arquivo, teste ou ponto de entrada é identificado; portanto, o escopo da implementação e a validação precisam de orientação dos maintainers.

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