example: Remove unnecessary guard

[jpmckinney]

Proposed Changes

self._channel in the cancel callback will have been initialized by the channel open callback (and will not have been nulled by the connection close callback).

Types of Changes

• Bugfix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (correction or otherwise)
• Cosmetics (whitespace, appearance)

Checklist

• I have read the CONTRIBUTING.md document
• All tests pass locally with my changes
• (N/A) I have added tests that prove my fix is effective or that my feature works
• (N/A) I have added necessary documentation (if appropriate)