docs: remove broken Code Climate GPA badge

[WuMingDao]

This PR removes the Code Climate GPA badge from the README. The badge currently displays a "no longer available" error because Code Climate has deprecated the GPA metric in favor of Maintainability grades, and the legacy link for this project is no longer functional. Removing it improves the documentation quality by eliminating dead assets.

Checklist

• only relevant code is changed (make a diff before you submit the PR)
• run tests npm run test
• tests are included
• commit message and code follows the Developer's Certification of Origin

Checklist (for documentation change)

• only relevant documentation part is changed (make a diff before you submit the PR)
• motivation/reason is provided
• commit message and code follows the Developer's Certification of Origin

[coveralls]

coverage: 97.682%. remained the same
when pulling e4d1e38 on WuMingDao:patch-1
into a33d985 on i18next:master.