Remove valid replication attribute `BackupRetentionPeriod` on `SourceDBInstanceIdentifier` instances

[tnielsen2]

I cannot enable log retention on an RDS instance that is confirmed as a Read Replica using the SourceDBInstanceIdentifier due to a troposphere error.

Error message

ValueError: BackupRetentionPeriod properties can't be provided when SourceDBInstanceIdentifier is present AWS::RDS::DBInstance.

When attempting to turn on retention to enable binlog replication to a 3rd party (non-aws resource), AWS support states that I must enable the BackupRetentionPeriod attribute on my RDS instance that is using the SourceDBInstanceIdentifier attribute.

Troposphere has a validation check that incorrectly asserts that BackupRetentionPeriod is not a valid property, while I can push this Cloudformation configuration to my AWS environment:

"exampleRDSInstance": {
  "Properties": {
    "AllocatedStorage": "4000",
    "AllowMajorVersionUpgrade": false,
    "AutoMinorVersionUpgrade": true,
    "CopyTagsToSnapshot": true,
    "DBInstanceClass": "db.m6i.large",
    "DBInstanceIdentifier": "omitted-rr",
    "DBParameterGroupName": {
      "Ref": "omitted"
    },
    "Engine": "mysql",
    "EngineVersion": "8.0.28",
    "Iops": 15000,
    "LicenseModel": "general-public-license",
    "Port": "3306",
    "BackupRetentionPeriod": "7",
    "PreferredMaintenanceWindow": "wed:09:02-wed:09:32",
    "SourceDBInstanceIdentifier": {
      "Fn::ImportValue": "omitted"
    },
    "StorageEncrypted": true,
    "StorageType": "io1",
    "Tags": [omitted]
  },
  "Type": "AWS::RDS::DBInstance"
},

This pull request removes the invalid assertion that BackupRetentionPeriod is not compatible with SourceDBInstanceIdentifier.

Documentation does not state this as an incompatible attribute.

Don't set the BackupRetentionPeriod, DBName, MasterUsername, MasterUserPassword, and PreferredBackupWindow properties. The database attributes are inherited from the source DB instance, and backups are disabled for read replicas.