Add callback for fs.unlink()

[deisterhold]

Add required callback for fs.unlink() even if the result is not checked.
Change-type: patch

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[ghost]

An error occurred whilst building your landr site preview:

        {
  "name": "TypeError",
  "message": "Cannot read property 'toLowerCase' of undefined",
  "stack": "TypeError: Cannot read property 'toLowerCase' of undefined\n    at /usr/src/app/node_modules/scrutinizer/lib/utils/markdown.js:115:32\n    at Array.some (<anonymous>)\n    at /usr/src/app/node_modules/scrutinizer/lib/utils/markdown.js:114:21\n    at Array.findIndex (<anonymous>)\n    at getMarkdownSection (/usr/src/app/node_modules/scrutinizer/lib/utils/markdown.js:111:37)\n    at async getHighlights (/usr/src/app/node_modules/scrutinizer/lib/utils/markdown.js:152:19)\n    at async module.exports (/usr/src/app/node_modules/scrutinizer/lib/plugins/readme-sections.js:90:17)"
}