revert(blog): drop English language — back to Chinese-only

Tried bilingual with the giscus-debug post translated, but the
language switcher wasn't actually wiring up correctly (English link
went to /en/ instead of /en/posts/<slug>/), and one English post
isn't enough to justify the complexity.

Changes:
- hugo.yaml: remove 'en' block under languages
- delete content/en/ entirely

The English version of the new post is preserved in git history
(commit f0c3bd8) if we ever want to revive bilingual.

Author: lzm

content/en/_index.md

@@ -9,27 +9,18 @@ title: LiMing编程笔记
 defaultContentLanguage: zh-cn
 defaultContentLanguageInSubdir: false
 
-# ---------- 多语言 ----------
-# 中文为默认语言，内容在 /posts/...（URL 不变）；英文内容放 content/en/...，
-# 渲染到 /en/posts/...。翻译某个 post 时新建同名 *.en.md 并加相同的
-# translationKey，PaperMod 的 language toggle 会自动关联。
+# ---------- 单语（中文） ----------
+# 之前试过加英文（content/en/）但语言切换器没真正起效，
+# 而且只有一篇英文 post，没意义。后续如果再开双语，
+# 把这段注释换回多语言配置 + 重建 content/en/ 即可。
 languages:
   zh-cn:
     languageName: 简体中文
     weight: 1
     languageCode: zh-CN
     title: "LiMing编程笔记"
     params:
-      displayFullLangName: true
       description: "技术学习笔记 · 实战 · 调优"
-  en:
-    languageName: English
-    weight: 2
-    languageCode: en-US
-    title: "LiMing's Coding Notes"
-    params:
-      displayFullLangName: true
-      description: "Tech learning notes · practical · tuned"
 
 # ---------- Taxonomies ----------
 # Hugo 默认是 categories + tags；显式列出以保留默认并新增 series。