fix: Remove the unused edx-custom-a11y-rules package.

This package was only used by bok-choy which was archived a while ago.
We just missed the fact that this package was also a node dependency.

See openedx/public-engineering#13 for more
details.

package.json

@@ -89,7 +89,6 @@
     "@edx/mockprock": "github:openedx/mockprock#3ad18c6888e6521e9bf7a4df0db6f8579b928235",
     "@edx/stylelint-config-edx": "2.3.3",
     "babel-jest": "26.0.0",
-    "edx-custom-a11y-rules": "1.0.6",
     "enzyme": "3.11.0",
     "enzyme-adapter-react-16": "1.15.8",
     "eslint-import-resolver-webpack": "0.8.4",