Remove undo/redo formatting toolbar buttons

There have been long standing problems with the buttons not working in
Firefox, and now a [further
problem](https://groups.google.com/d/msg/tiddlywiki/GlsruQyPOag/BAhnI2mt
BgAJ) has come to light, and been similarly hard to resolve.

On balance, I’ve decided to remove the undo/redo buttons from the
toolbar (the ctrl/cmd-Z shortcut is still operational), thus avoiding
the problems, and saving some space in the core.

core/modules/editor/engines/framed.js

@@ -196,31 +196,6 @@ FramedEngine.prototype.executeTextOperation = function(operation) {
 	return newText;
 };
 
-/*
-Execute a command
-*/
-FramedEngine.prototype.execCommand = function(command) {
-	var msg = "Warning: the '" + command + "' button does not work in Firefox without installing the CodeMirror plugin.\n\n(Standard operating system keyboard shortcuts will work correctly)";
-	this.iframeNode.focus();
-	this.domNode.focus();
-	switch(command) {
-		case "undo":
-			if($tw.browser.isFirefox) {
-				alert(msg);
-			} else {
-				this.iframeDoc.execCommand("undo",false,null);
-			}
-			break;
-		case "redo":
-			if($tw.browser.isFirefox) {
-				alert(msg);
-			} else {
-				this.iframeDoc.execCommand("redo",false,null);
-			}
-			break;
-	}
-};
-
 exports.FramedEngine = FramedEngine;
 
 })();

core/modules/editor/operations/text/redo.js

@@ -1,19 +0,0 @@
-/*\
-title: $:/core/modules/editor/operations/text/redo.js
-type: application/javascript
-module-type: texteditoroperation
-
-Text editor operation to tell the browser to perform an redo
-
-\*/
-(function(){
-
-/*jslint node: true, browser: true */
-/*global $tw: false */
-"use strict";
-
-exports["redo"] = function(event,operation) {
-	this.execCommand("redo");
-};
-
-})();

core/modules/editor/operations/text/undo.js

@@ -1,19 +0,0 @@
-/*\
-title: $:/core/modules/editor/operations/text/undo.js
-type: application/javascript
-module-type: texteditoroperation
-
-Text editor operation to tell the browser to perform an undo
-
-\*/
-(function(){
-
-/*jslint node: true, browser: true */
-/*global $tw: false */
-"use strict";
-
-exports["undo"] = function(event,operation) {
-	this.execCommand("undo");
-};
-
-})();