Great_expectations: The timestamp on slack messages does not correspond with the validation run time.

[Aylr]

Describe the bug
The timestamp on slack messages does not correspond with the validation run time.

To Reproduce
Steps to reproduce the behavior:

1. Run a validation operator that has a slack action
2. Find the slack message
3. Note the discrepancy

Expected behavior
I expect the timestamp to either be the same as the run ID run time or be removed since it is now obsolete?

Environment (please complete the following information):

• OS: [e.g. iOS] macOS
• GE Version: [e.g. 0.10.0] 0.11.8

Additional context
Here are two messages where it's obvious the timestamp is not correct. It looks like it might be the time the slack message was sent.

[Shinnnyshinshin]

@Aylr you're right, the Timestamp is calculated as the time that the Slack message is sent. I'm ok with removing the field entirely, since the run_time provides information on when the Validation ran, and Slack already has a timestamp associated with each message. I'll try to package this change with the a Slack-related PR I've been working on.

[Shinnnyshinshin]

closed by #1707