Remove yarn.lock

[lauriii]

Even if we use yarn internally, from the basic starter users perspective there's no reason to require using yarn. We can remove yarn.lock from the basic starter and the lock file will be created when the project is created from the starter kit.

[lauriii]

Steps for testing this:

• Ensure yarn is installed on your environment
• Run npx create-next-app -e https://github.com/acquia/next-acms/tree/remove-yarn-lock/starters/basic-starter and confirm that yarn is used for installing and yarn.lock exists after the command has run
• Run npx create-next-app -e https://github.com/acquia/next-acms/tree/remove-yarn-lock/starters/basic-starter --use-npm and ensure npm is used for installing and package-lock.json exists but yarn.lock doesn't exist

[harumijang]

Tested manually, works as expected