Remove dead weight code

[maxime-rainville]

As a CMS Squad, we don't want to carry forward code that is not needed to minimise our maintenance burden.

Acceptance criteria

• [x] JSONField is folded back into LinkField.
• [x] DBJson and DBLink are removed.
• [x] JsonData is removed and all reference to it are substituted.
• [x] Module is still broadly functional.

PR

• https://github.com/silverstripe/silverstripe-linkfield/pull/118

[emteknetnz]

Linked PR has been merged