fix(aws-s3): remove policy argument (#730)

This argument served as a red herring, and made it harder
for people to find the correct method of setting a bucket
policy, by calling `addToPolicy()`.

The provided method of providing an existing Policy object on object
creation cannot actually work, because a `BucketPolicy` object requires
the `Bucket` instance as a constructor argument, which means the
`Bucket` object cannot take a `BucketPolicy` in its constructor.

Fixes #672.

packages/@aws-cdk/aws-s3/lib/bucket.ts

@@ -323,14 +323,6 @@ export interface BucketProps {
      */
     removalPolicy?: cdk.RemovalPolicy;
 
-    /**
-     * The bucket policy associated with this bucket.
-     *
-     * @default A bucket policy will be created automatically in the first call
-     * to addToPolicy.
-     */
-    policy?: BucketPolicy;
-
     /**
      * Whether this bucket should have versioning turned on or not.
      *
@@ -379,7 +371,6 @@ export class Bucket extends BucketRef {
         cdk.applyRemovalPolicy(resource, props.removalPolicy);
 
         this.versioned = props.versioned;
-        this.policy = props.policy;
         this.encryptionKey = encryptionKey;
         this.bucketArn = resource.bucketArn;
         this.bucketName = resource.ref;