Remove explicit ensure stop - it breaks graceful shutdown.

[samuel-williams-shopify]

ensure stop breaks graceful shutdown as stop gets delivered to child tasks twice, causing defer_stop to immediately raise instead of waiting.

Types of Changes

• Bug fix.

Contribution

• I added tests for my changes.
• I tested my changes locally.
• I agree to the Developer's Certificate of Origin 1.1.

[samuel-williams-shopify]

This will break sus-fixtures-async-http until we fix it in a better way.