Drop unused test run script for attr_encrypted

What does this MR do and why?

Drop unused test run script for attr_encrypted

We vendored this gem a while ago, and it contained a lot of code to deal with older versions of Ruby and Rails that are not relevant to use.

This test run script is not used either, we just call straight into rspec in CI/CD via bundler.

Let's drop this since it's leading to issue reports.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #426036 (closed)