MAINT: remove internal cert

[jamespollard8]

We believe the internal cert is no longer necessary.

Next steps:

• configure "plaintext" (Istio handled) via overlays instead of editing "vendired" files

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WHAT is this change about?

Ripping out the internal certificate if it's no longer needed.

Does this PR introduce a change to config/values.yml?

Yes. I'm not sure if removing a value is a breaking change.

Acceptance Steps

1. PR tests pass
2. When I review the codebase, I see no more references to the internal certificate

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[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/176372188

The labels on this github issue will be updated when the story is started.

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[Birdrock]

@jamespollard8 Looks good to me. Squash/merge some of the commits to tidy up the history and I'd say it's ready.