Remove promise jsdoc (#641)

This patch removes unnecessary "promise" return types in jsdoc for async functions.

lib/Input.js

@@ -29,7 +29,6 @@ class Keyboard {
   /**
    * @param {string} key
    * @param {{text: (string|undefined)}} options
-   * @return {!Promise}
    */
   async down(key, options = {}) {
     const text = options.text;
@@ -65,7 +64,6 @@ class Keyboard {
 
   /**
    * @param {string} key
-   * @return {!Promise}
    */
   async up(key) {
     this._modifiers &= ~this._modifierBit(key);
@@ -80,7 +78,6 @@ class Keyboard {
 
   /**
    * @param {string} char
-   * @return {!Promise}
    */
   async sendCharacter(char) {
     await this._client.send('Input.dispatchKeyEvent', {