feat(configuration): remove the ability to externally configure testslowimport

mentioned in commit 0176596e

mentioned in merge request gitlab-org/build/CNG!1176 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!2235 (merged)

mentioned in merge request omnibus-gitlab!6489 (merged)

mentioned in merge request omnibus-gitlab!6490 (merged)

mentioned in merge request gitlab-org/build/CNG!1185 (merged)

mentioned in merge request gitlab-org/build/CNG!1192 (merged)

mentioned in merge request gitlab-org/charts/gitlab!2845 (merged)

mentioned in merge request gitlab-development-kit!2964 (merged)