[ci skip] remove outdated externs file

externs/closure-upstream-externs.js

@@ -1,74 +0,0 @@
-/**
- * @fileoverview Externs to upstream to closure compiler
- * @externs
- */
-
-/** @record */
-function CustomElement(){}
-/** @type {!Array<string>|undefined} */
-CustomElement.observedAttributes;
-/** @type {function()|undefined} */
-CustomElement.prototype.connectedCallback;
-/** @type {function()|undefined} */
-CustomElement.prototype.disconnectedCallback;
-/** @type {function(string, ?string, ?string, ?string)|undefined} */
-CustomElement.prototype.attributeChangedCallback;
-
-/** @type {boolean} */
-Event.prototype.composed;
-
-/**
- * @return {!Array<!(Element|ShadowRoot|Document|Window)>}
- */
-Event.prototype.composedPath = function(){};
-
-/**
- * @param {!{mode: string}} options
- * @return {!ShadowRoot}
- */
-HTMLElement.prototype.attachShadow = function(options){};
-
-/**
- * @constructor
- * @extends {HTMLElement}
- */
-function HTMLSlotElement(){}
-
-/**
- * @param {!{flatten: boolean}=} options
- * @return {!Array<!Node>}
- */
-HTMLSlotElement.prototype.assignedNodes = function(options){};
-
-/** @type {HTMLSlotElement} */
-Node.prototype.assignedSlot;
-
-/** @constructor */
-function InputDeviceCapabilities(){}
-
-/** @type {boolean} */
-InputDeviceCapabilities.prototype.firesTouchEvents;
-
-/** @type {InputDeviceCapabilities} */
-MouseEvent.prototype.sourceCapabilities;
-
-const customElements = {
-  /**
-   * @param {string} tagName
-   * @param {!CustomElement} klass
-   * @param {Object=} options
-   * @return {!CustomElement}
-   */
-  define(tagName, klass, options){},
-  /**
-   * @param {string} tagName
-   * @return {CustomElement}
-   */
-  get(tagName){},
-  /**
-   * @param {string} tagName
-   * @return {Promise<!CustomElement>}
-   */
-  whenDefined(tagName){}
-}
-window.customElements = customElements;