(feat) O3-2353: Use demo data content package

[NethmiRodrigo]

This PR updates the reference application to use the demo data content package. Here's what I did:

1. Removed the backend configurations that will get introduced by the demo data content package.
2. Updated the distro properties file to include the demo data content package.
3. @jayasanka-sack created a bamboo plan for the demo ref app content package - https://ci.openmrs.org/browse/CH-CONREFD

Concerns

This leaves the frontend config (found in config-core_demo.json). These frontend configurations are already available in the reference application content package and the demo reference application content package. We should figure out a way to remove the frontend configuration from here and have it from the content package itself.

[jayasanka-sack]

Thanks @NethmiRodrigo! Let's address the frontend config in a separate PR.