cleanup: remove the legacy mask editor and all related code (#7370)

## Summary

Removes the legacy mask editor. May also want to remove the "Beta" tags
on all the current mask editor components/mentions in the app now as
well.

## Context

Telemetry data shows zero users using the legacy mask editor. It has
been considerable time since we switched to the new one, and there
really is no reason to use the legacy version given how lacking it is.

In https://github.com/Comfy-Org/ComfyUI_frontend/pull/7332 (v1.35.2 -
Dec 11, 2025), we added a final warning that the legacy mask editor is
being removed.

On 1.36, this PR can be merged, as more than enough warning will have
been given.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7370-cleanup-remove-the-legacy-mask-editor-and-all-related-code-2c66d73d365081d58fbed0f3c84bcb0d)
by [Unito](https://www.unito.io)

build/plugins/comfyAPIPlugin.ts

@@ -9,11 +9,7 @@ interface ShimResult {
 const SKIP_WARNING_FILES = new Set(['scripts/app', 'scripts/api'])
 
 /** Files that will be removed in v1.34 */
-const DEPRECATED_FILES = [
-  'scripts/ui',
-  'extensions/core/maskEditorOld',
-  'extensions/core/groupNode'
-] as const
+const DEPRECATED_FILES = ['scripts/ui', 'extensions/core/groupNode'] as const
 
 function getWarningMessage(
   fileKey: string,