[mergify] remove backport-skip if added a backport label

[v1v]

What is the problem this PR solves?

Keep tidy the GItHub labels in the PRs.

How does this PR solve the problem?

Remove backport-skip label when the backport labels have been added.

How to test this PR locally

NA

Test

v1v/poc-bump-automation#13

Related issues

Similar to elastic/beats#28235

[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-7.x is the label to automatically backport to the 7.x branch.
• backport-7./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[apmmachine]

💚 Build Succeeded

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Build stats

• Start Time: 2021-10-05T11:01:26.320+0000

• Duration: 46 min 23 sec

• Commit: e3e3e10

Test stats 🧪

Test	Results
Failed	0
Passed	6141
Skipped	14
Total	6155

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