destination/credential datasources removed from tf provider

[log1cb0mb]

With reference to issue reported here:
infrahq/terraform-provider-infra#16

Why? :(

[mxyng]

Summary from the linked issue:

The infra_destination and infra_credential data sources were removed because there wasn't a strong use case at the time. At the same time, the data source is limited by the credentials API resource which can lead to unexpected errors when created as a Terraform data source. Once we've sorted out this issue, we'll re-evaluate the fate of these data sources.