Remove iron-input polymer dependency

Replace the iron-input wrapper and its bind-value attribute with a
plain HTML input element using Lit's .value property binding.

iron-input is a Polymer component that no longer renders properly
in Gerrit v3.13 as part of the ongoing migration away from Polymer.

Change-Id: Iee0a15a2d430cae9bd0c6617cce50a6213b2a3b8

ref-copy/ref-copy.js

diff --git a/ref-copy/ref-copy.js b/ref-copy/ref-copy.js
index 46fc39f..efb3a35 100644
--- a/ref-copy/ref-copy.js
+++ b/ref-copy/ref-copy.js
@@ -37,16 +37,12 @@
   render() {
     return html`
     <div class="text">
-      <iron-input
-          bind-value="${this.revision.ref}"
-          readonly>
-        <input
-            id="input"
-            is="iron-input"
-            type="text"
-            @click="${this._handleInputTap}"
-            readonly/>
-      </iron-input>
+      <input
+          id="input"
+          type="text"
+          .value="${this.revision.ref}"
+          @click="${this._handleInputTap}"
+          readonly/>
       <gr-button
         id="button"
         link