remove webpack.alias.js from Contribution.md

[mkcy3]

What does it do?

Updates the contribution.md file to equal the online contribution docs.

Why is it needed?

If you visit the design-system and use the contributing docs, it will tell you to remove the package in the webpack config, which is no longer necessary.

https://contributor.strapi.io/guides/working-with-the-design-system

https://github.com/strapi/design-system/blob/fcdf1f2b88fce49c79691ee84334b2ffccc8d61e/CONTRIBUTING.md?plain=1#L179

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