Remove "dependency_list" feature flag

there is a list of feature flags enabled on production but not used in code https://gitlab.com/gitlab-org/gitlab-ee/issues/13567

dependency_list flag is still checked with project.feature_available?(:dependency_list) but really we don't need it anymore.

Removing this flag from production caused 404 error for /dependencies page. So we need to carefully remove this flag and its checks in the codebase.

/cc @markrian @gonzoyumo