mergify: remove duplicated config and use the default ones for the backport labels

[v1v]

Release notes

[rn:skip]

What does this PR do?

Tidy up duplicated configuration and honour the defaults from #17551

Why is it important/What is the impact to the user?

Avoid duplicated configuration entries
See #17740 (comment)

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files (and/or docker env variables)
• I have added tests that prove my fix is effective or that my feature works

Author's Checklist

• [ ]

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🤖 GitHub comments

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Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[mergify]

This pull request does not have a backport label. Could you fix it @v1v? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• If no backport is necessary, please add the backport-skip label

[donoghuc]

Thanks!