Clean legacy SimpleMDE code (#17926)

Since we are using EasyMDE now, we do not need to keep the SimpleMDE code anymore.

This PR removes all legacy SimpleMDE code, and makes some related changes:
* `createCommentEasyMDE` can accept native DOM element, and it doesn't need `jQuery.data` to store EasyMDE editor object (as discussed about the frontend guideline).
* introduce `getAttachedEasyMDE` to get the attached EasyMDE editor object, it's easier to find all the usage of EasyMDE.
* rename variable names from `$simplemde` to `easyMDE`, the `$` was incorrect because it is a EasyMDE editor, not a jQuery object.

With this PR, it will be easier to do more refactoring or replacing EasyMDE with other editors.

routers/web/repo/commit.go

@@ -253,7 +253,7 @@ func FileHistory(ctx *context.Context) {
 func Diff(ctx *context.Context) {
 	ctx.Data["PageIsDiff"] = true
 	ctx.Data["RequireHighlightJS"] = true
-	ctx.Data["RequireSimpleMDE"] = true
+	ctx.Data["RequireEasyMDE"] = true
 	ctx.Data["RequireTribute"] = true
 
 	userName := ctx.Repo.Owner.Name