Remove unused Instagram service settings

[jmooring]

We stopped using these Instagram service settings a year ago in v0.123.0:

[services.instagram]vice
  accessToken = ''
  disableInlineCSS = false

I'd like to remove them because of their presence in the docs.yaml file. They no longer belong in the documentation, and I dislike manually creating config examples because we have to remember to update them in the future, which we won't.

We could either:

1. Silently remove them
2. Log deprecation messages

INFO deprecated: the services.instagram.accessToken configuration setting was deprecated in Hugo 0.144.0 and will be removed in a future release. This setting is no longer used by the embedded Instagram shortcode. You may remove it from your site configuration."

I can think of two (somewhat flimsy) reasons to log deprecation messages:

1. There's a very small chance that someone is using these settings with their custom Instagram shortcode
2. It would give users notice to clean up their site config

I'd prefer to just silently remove the settings and be done with it. Anyone who has a custom shortcode that uses those settings will see an informative error (can't evaluate field Foo in type services.Instagram).