remove the target_name options from the elastic rake tasks

The following discussion from !50817 (merged) should be addressed:

• @DylanGriffith started a discussion:

  We should remove the target_name options from the rake task unless we need them. These are confusing since these tasks are needed to manage multiple indices now and it no longer makes sense to give the target name of just one index. It could also cause confusion. I believe these separate target name concepts don't need to be used in rake tasks anymore now that we support reindexing through the UI.