CLN: avoid use of client.dataset, prefer DatasetReference constructor, instead

[tswast]

Getting deprecation warnings in integration tests:

samples/tests/test_to_gbq.py::test_to_gbq_simple
  /root/project/pandas_gbq/load.py:59: PendingDeprecationWarning: Client.dataset is deprecated and will be removed in a future version. Use a string like 'my_project.my_dataset' or a cloud.google.bigquery.DatasetReference object, instead.
    destination_table = client.dataset(dataset_id).table(table_id)

[nathan-ok]

A comment i found within the client.py class in the dataset function relating to this issue,
As of google-cloud-bigquery version 1.7.0, all client methods
that take a
:class:~google.cloud.bigquery.dataset.DatasetReference or
:class:~google.cloud.bigquery.table.TableReference also take a
string in standard SQL format, e.g. project.dataset_id or
project.dataset_id.table_id.
https://cloud.google.com/bigquery/docs/reference/rest/v2/datasets#datasetreference
https://cloud.google.com/bigquery/docs/reference/rest/v2/tables#tablereference