Clean up of legacy addthis references, removed reference to old tag fiel...

## Ready State: Yes|No

Relates to Jira bug: https://imagex.atlassian.net/browse/LDC-334

I noticed reference to addThis & tag fields which are no longer part of the installkit.
See: 
https://github.com/imagex/imagex_group/commit/8ba38954903dd62ec1ee6992965e26e02d93d239
https://github.com/imagex/imagex_group/commit/bb745819b944d404f22f7b0a1e8f7307156e2ad6

## Requirements & Dependencies

None

## Deployment Instructions

* Merge code.
* Enable module `imagex_group`
* Clear caches with `drush cache-clear all`

## Usage/Test Instructions

* Ensure no reference to field_tag or field_addthis remain.