Pindexer remove unused appviews (#4910)

## Describe your changes

This removes:
- The Dex AppView
- The ShieldedPool AppView

The Dex AppView shouldn't be needed, given that we have an app view
specific to the dex explorer now.

The shielded pool app view was just an example, which can be removed
given the many other examples now.

Other AppViews which should be removed in a later PR after making sure
they're not used:
- the supply AppView
  - superceded by the insights app view, but is known to be used
- the ibc AppView
  - I think there's ongoing development using this, potentially?
  
## Testing

The other app views should continue to work.

## Checklist before requesting a review

- [x] I have added guiding text to explain how a reviewer should test
these changes.

- [x] If this code contains consensus-breaking changes, I have added the
"consensus-breaking" label. Otherwise, I declare my belief that there
are not consensus-breaking changes, for the following reason:

  > indexing only