Remove workaround in gemnasium-gradle-plugin for making Dependency Scanning work

Summary

In order to make Dependency Scanning work for gemnasium-gradle-plugin, a workaround was added to remove an analyzable (manual-test) directory in the before_script. This is a rather brittle solution and ought to be removed when the finders package under gemnasium supports DS_EXCLUDED_PATHS: #292457 (closed)

Improvements

Remove before_script section.

Risks

None as MR pipeline will immediately show whether change is working.

Involved components

https://gitlab.com/gitlab-org/security-products/analyzers/gemnasium