fix(toolkit-lib): remove unused extraUserAgent option from WatchOptions (#314)

BREAKING CHANGE: The `extraUserAgent`option in `WatchOptions` interface
was unused and had no effect on functionality. This change removes the
unused parameter.

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

Author: mrgrain

packages/@aws-cdk/toolkit-lib/lib/actions/watch/index.ts

@@ -1,13 +1,6 @@
 import type { BaseDeployOptions } from '../deploy/private';
 
 export interface WatchOptions extends BaseDeployOptions {
-  /**
-   * The extra string to append to the User-Agent header when performing AWS SDK calls.
-   *
-   * @default - nothing extra is appended to the User-Agent header
-   */
-  readonly extraUserAgent?: string;
-
   /**
    * Watch the files in this list
    *