fix(toolkit-lib): remove AsyncDisposable from Toolkit (#312)

BREAKING CHANGE: Remove the `AsyncDisposable` interface and `dispose()`
method from the `Toolkit` class. These were previously unused and had no
effect.

---
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under the terms of the Apache-2.0 license

Author: mrgrain

packages/@aws-cdk/toolkit-lib/lib/toolkit/toolkit.ts

@@ -81,7 +81,7 @@ export interface ToolkitOptions {
 /**
  * The AWS CDK Programmatic Toolkit
  */
-export class Toolkit extends CloudAssemblySourceBuilder implements AsyncDisposable {
+export class Toolkit extends CloudAssemblySourceBuilder {
   /**
    * The toolkit stack name used for bootstrapping resources.
    */
@@ -114,14 +114,6 @@ export class Toolkit extends CloudAssemblySourceBuilder implements AsyncDisposab
     this.ioHost = withTrimmedWhitespace(ioHost);
   }
 
-  public async dispose(): Promise<void> {
-    // nothing to do yet
-  }
-
-  public async [Symbol.asyncDispose](): Promise<void> {
-    await this.dispose();
-  }
-
   /**
    * Access to the AWS SDK
    */