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)

src/locales/zh/settings.json

@@ -181,10 +181,6 @@
     "name": "将画笔调整锁定到主轴",
     "tooltip": "启用后，画笔调整将仅根据您移动的方向影响大小或硬度。"
   },
-  "Comfy_MaskEditor_UseNewEditor": {
-    "name": "使用新画笔编辑器",
-    "tooltip": "切换到新的画笔编辑器界面"
-  },
   "Comfy_ModelLibrary_AutoLoadAll": {
     "name": "自动加载所有模型文件夹",
     "tooltip": "开启后，打开模型库会加载所有文件夹内的模型（可能导致卡顿）。关闭后，仅加载当前文件夹内的模型。"