chore: remove Neo4j backend implementation

[gaganhr94]

Description of the PR

Changes to remove support for Neo4j and delete all the code related to it.

Fixes #3053

PR Checklist

• All commits have a Developer Certificate of Origin (DCO) -- they are generated using -s flag to git commit.
• All new changes are covered by tests
• If GraphQL schema is changed, make generate has been run
• If GraphQL schema is changed, GraphQL client updates/additions have been made
• If OpenAPI spec is changed, make generate has been run
• If ent schema is changed, make generate has been run
• If collectsub protobuf has been changed, make proto has been run
• All CI checks are passing (tests and formatting)
• All dependent PRs have already been merged

[gaganhr94]

CI failing due to known issue. Will be fixed in #3045