API Remove silverstripe/campaign-admin integration support

[emteknetnz]

Issue silverstripe/.github#342

[GuySartorelli]

i18n._t('CampaignAdmin.DELETECAMPAIGN', 'Are you sure you want to delete this record?') in client/src/components/GridField/GridField.js should not use a localisation key from campaign admin. There's a few others in that file too.

1. Double check if that is actually at all related to campaign functionality and if so, update the logic as needed
2. Use a localisation key that doesn't have anything to do with campaigns

[emteknetnz]

I don't actually know what this react component for GridField even does, or if it's even used (I suspect it isn't)

Perhaps this one should be a new card to work out if we should just delete the file, along with the seemingly associated GridFieldActions.js?

[GuySartorelli]

Looks like Gridfield.js is specifically the component for the gridfield-like UI in campaign admin. Almost certainly not used anywhere else in core.

That said, if you want to remove that in a separate card I'm okay with that. Up to you.

Either way, this needs a rebase.

[emteknetnz]

Will do as new card

[GuySartorelli]

LGTM