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/ui/EditorToolbar/redo.tid

@@ -1,11 +0,0 @@
-title: $:/core/ui/EditorToolbar/redo
-tags: $:/tags/EditorToolbar
-icon: $:/core/images/redo
-caption: {{$:/language/Buttons/Redo/Caption}}
-description: {{$:/language/Buttons/Redo/Hint}}
-condition: [<targetTiddler>!is[image]]
-
-<$action-sendmessage
-	$message="tm-edit-text-operation"
-	$param="redo"
-/>

core/ui/EditorToolbar/undo.tid

@@ -1,12 +0,0 @@
-title: $:/core/ui/EditorToolbar/undo
-tags: $:/tags/EditorToolbar
-icon: $:/core/images/undo
-caption: {{$:/language/Buttons/Undo/Caption}}
-description: {{$:/language/Buttons/Undo/Hint}}
-condition: [<targetTiddler>!is[image]]
-button-classes: tc-text-editor-toolbar-item-start-group
-
-<$action-sendmessage
-	$message="tm-edit-text-operation"
-	$param="undo"
-/>