Remove helpers introduced for Istio migration

There were a number of helpers introduced into umbrella to facilitate the transition to operatorless Istio, which supported both deployment methods simultaneously.

Now that we've made the switch we should go through and consider cleaning up any that no longer serve a purpose to avoid any future confusion.

These are the ones I have been tracking https://repo1.dso.mil/big-bang/bigbang/-/blob/master/chart/templates/_helpers.tpl?ref_type=heads#L491-562.

Generally they include a check for .Values.istio.enabled which is no longer valid and the change should be removing the old branch logic for scenarios where .Values.istio.enabled=true or removing the helper entirely.