remove the default cluster disk size of 2gb

This default value changes by the size of the cluster as specified in the api docs-
```
Each instance size has its own default value. If you set
a value below the instance default, Atlas replaces it with the default
value.
```
more info here:
https://docs.atlas.mongodb.com/reference/api/clusters-create-one/

Author: akshaykarle

mongodbatlas/cluster.go

@@ -66,7 +66,6 @@ func resourceCluster() *schema.Resource {
 			"disk_size_gb": &schema.Schema{
 				Type:     schema.TypeFloat,
 				Optional: true,
-				Default:  2,
 			},
 			"replication_factor": &schema.Schema{
 				Type:     schema.TypeInt,