Remove old core mongodb db

[sevivenegas]

References

• Linear: Remove Old Core MongoDB Database

Proposed Changes

• local.go: delete references to "core" when setting up local database
• helpers.go: delete UpdateUser and FindUser
• main.go: delete the FindUser reference (keep the second half of the condition)

[sevivenegas]

@jfmath04

[jfmath04]

Code changes look good! Let's remove the .DS_Store file and the deletion of .env.local.txt from the PR. Also, we don't use yarn for the backend, so let's remove the yarn files too. The only changes should be in the go files you have here.