revert: use table clone instead of system time for read_gbq_table (#109)

[tswast]

This reverts commit 031f253.

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> 🦕

[tswast]

For context, the reason we reverted #109 is that we very quickly hit the 1000 table clones per table limit. https://cloud.google.com/bigquery/quotas#table_clones