Remove unnecessary clipboard clear button

This button fills no real purpose. It's easy to mark everything and
delete with either "Ctrl + A -> Delete" or, on touch devices, "long
press -> mark everything -> Delete".

app/ui.js

@@ -337,8 +337,6 @@ const UI = {
             .addEventListener('click', UI.toggleClipboardPanel);
         document.getElementById("noVNC_clipboard_text")
             .addEventListener('change', UI.clipboardSend);
-        document.getElementById("noVNC_clipboard_clear_button")
-            .addEventListener('click', UI.clipboardClear);
     },
 
     // Add a call to save settings when the element changes,
@@ -968,11 +966,6 @@ const UI = {
         Log.Debug("<< UI.clipboardReceive");
     },
 
-    clipboardClear() {
-        document.getElementById('noVNC_clipboard_text').value = "";
-        UI.rfb.clipboardPasteFrom("");
-    },
-
     clipboardSend() {
         const text = document.getElementById('noVNC_clipboard_text').value;
         Log.Debug(">> UI.clipboardSend: " + text.substr(0, 40) + "...");

vnc.html

@@ -142,9 +142,6 @@ <h1 class="noVNC_logo" translate="no"><span>no</span><br>VNC</h1>
                     <img alt="" src="app/images/clipboard.svg"> Clipboard
                 </div>
                 <textarea id="noVNC_clipboard_text" rows=5></textarea>
-                <br>
-                <input id="noVNC_clipboard_clear_button" type="button"
-                    value="Clear" class="noVNC_submit">
             </div>
             </div>