Remove deprecated pre-TGM Contentful lib functions

[aaronschachter]

What's this PR do?

• Removes lib/contentful functions no longer used in production: fetchBroadcast,fetchCampaign, fetchMessageForCampaignId, and renderMessageForPhoenixCampaign
• Renames parseDefaultAndOverrideContentfulCampaignsResponse as parseDefaultAndOverrideCampaignsResponse, and adds to exports for test coverage
• Renames fetchDefaultAndOverrideContentfulCampaignsForCampaignId as fetchDefaultAndOverrideCampaignsForCampaignId, adds test coverage for the middleware that calls it (lib/middleware/campaigns-single/contentful-campaigns)

How should this be reviewed?

Verify GET /campaigns and GET /campaigns/:id responses return expected data.

Any background context you want to provide?

• I'd like to deprecate the use of the Contentful campaign with campaignId set to default for defining default fields, and instead hardcode the default values to use if an optional field doesn't exist on the Campaign. Didn't make that change in this branch to help keep it reviewable.

• I'll be testing on staging by setting the Phoenix URL config variable to point to Ashes, putting Support Ashes and Phoenix URLs via config  #1019 on hold.

Relevant tickets

Cleanup work to prep for new Gambini types and fields in #1021

Checklist

• Tested on staging.

[aaronschachter]

I've set the Phoenix API config var back to Ashes, tested this on staging, and verified all API endpoints are working as expected.

[rapala61]

👍