Remove vtbackup Docker image

[frouioui]

Description

Turns out the vtbackup image has not been modified in years on git and that we never published its image on DockerHub, I am proposing to remove the related code to simplify our codebase. We can always revert this PR if we realize someone needs it.

[vitess-bot]

Review Checklist

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[frouioui]

This PR contradicts #14318, we can close it as soon as #14318 gets merged.