Drop unused canvass feature (#7067)

# Description & Issue number it closes 
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Please also include relevant motivation and context. -->

The canvass feature has missing database entries going back 20 years.

I don't see any open or closed issues about it, and a quick poll on
Gitter showed no one was using it.

## Screenshots (if appropriate)
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None.

## How to test the changes?

## Type of change

- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [x] Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [x] This change requires a documentation update

# How Has This Been Tested?

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Provide instructions so we can reproduce. Please also list any relevant
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# Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules