Remove database subnet

[p-j-smith]

I think we don't need both the private and database subnets - the database subnet can be used to expose the database to the public, but is not recommended.

We could create our own aws_db_subnet_group (using the private subnets), and use this when creating the RDS instance

[milanmlft]

I agree we don't need them. The only reason I added them is because the setup_vpc module failed when only enabling database_subnets without any private subnets. See terraform-aws-modules/terraform-aws-vpc#944.