Remove validation of RDS DB instance type names because Amazon frequently adds new ones

This PR removes the `choices` list of Amazon RDS DB instance types. Amazon frequently adds new ones, and if you try to use a newly added instance type that your Ansible distribution doesn't yet include, your deployment fails. E.g.:

```
TASK: [postgresql_rds | create PostgreSQL RDS database instance] ************** 
failed: [db-live] => {"failed": true}
msg: value of instance_type must be one of: db.t1.micro,db.m1.small,db.m1.medium,db.m1.large,db.m1.xlarge,db.m2.xlarge,db.m2.2xlarge,db.m2.4xlarge,db.m3.medium,db.m3.large,db.m3.xlarge,db.m3.2xlarge,db.cr1.8xlarge, got: db.r3.2xlarge
```

The only fix is to wait for an Ansible update that includes the new instance type, or fork/modify Ansible code.

I was the last committer to add to this list with this commit on March 3: https://github.com/ansible/ansible/commit/635fdcb5334a71b3ab798cbe358b8337f02f9314. Since then, new instance types have been added. Instead of continually updating the list in the RDS module, I propose we remove the hard-coded list altogether. This PR does that.