[DBSubnetGroup] Remove validation on DBSubnetGroupName (#537)

Co-authored-by: Angus Yuen <[email protected]>

Author: angusy29

aws-rds-dbsubnetgroup/aws-rds-dbsubnetgroup.json

@@ -7,8 +7,7 @@
       "type": "string"
     },
     "DBSubnetGroupName": {
-      "type": "string",
-      "pattern": "^(?!default$)[a-zA-Z]{1}[a-zA-Z0-9-_\\.\\s]{0,254}$"
+      "type": "string"
     },
     "SubnetIds": {
       "type": "array",

aws-rds-dbsubnetgroup/docs/README.md

@@ -49,8 +49,6 @@ _Required_: No
 
 _Type_: String
 
-_Pattern_: <code>^(?!default$)[a-zA-Z]{1}[a-zA-Z0-9-_\.\s]{0,254}$</code>
-
 _Update requires_: [Replacement](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/using-cfn-updating-stacks-update-behaviors.html#update-replacement)
 
 #### SubnetIds