Remove redundant methods from Body class

This patch:
- removes Body.arrayBuffer. This method is redundant since there's
  already a Body.buffer() method
- removes Body.bodyUsed getter.

References #106

lib/NetworkManager.js

@@ -156,13 +156,6 @@ class Body {
     return this._contentPromise;
   }
 
-  /**
-   * @return {boolean}
-   */
-  get bodyUsed() {
-    return !!this._contentPromise;
-  }
-
   /**
    * @return {!Promise<string>}
    */
@@ -178,14 +171,6 @@ class Body {
     let content = await this.text();
     return JSON.parse(content);
   }
-
-  /**
-   * @return {!Promise<!ArrayBuffer>}
-   */
-  async arrayBuffer() {
-    let content = await this.buffer();
-    return content.buffer;
-  }
 }
 helper.tracePublicAPI(Body);