Simplify Dependency List Vuex store

The following discussion from !31097 (merged) should be addressed:

• @farias-gl started a discussion: (+1 comment)

  Not sure how JS unit tests are currently passing with this removal.

  DEPENDENCY_LIST_TYPES.vulnerable

  Seems to be still used in:

  https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/spec/frontend/dependencies/store/actions_spec.js#L11

  https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/spec/frontend/dependencies/store/mutations_spec.js#L15

This is a continuation of #212867 (closed).

The Dependency List Vuex store contains the notion of list types, since in the previous version of the Dependency List, when there were literally two lists: one displaying "All" dependencies, and one displaying only "Vulnerable" dependencies (each available as tabs).

The latest design removed that notion, and now everything is in one list.

Therefore, some actions/mutations are no longer technically necessary to drive the UI - e.g., the addListType/ADD_LIST_TYPE action/mutation, and the getters which reduce over the runtime list types.

This issue is about removing that complexity.

I consider this very low priority, since the complexity overhead is minimal. Future feature work on the Dependency List would better indicate whether or not this would be worthwhile.