chore: remove undeveloped blog theme

BREAKING CHANGE: please head https://vuepress-theme-blog.ulivz.com/ to see the default blog theme
for vuepress

packages/@vuepress/theme-blog/.npmignore

-__tests__
-__mocks__
\ No newline at end of file

packages/@vuepress/theme-blog/README.md

-# @vuepress/theme-blog
-
-> theme-blog for vuepress (WIP)

packages/@vuepress/theme-blog/index.js

-module.exports = {
-  plugins: [
-    '@vuepress/blog',
-    '@vuepress/pagination',
-    '@vuepress/medium-zoom',
-    ['@vuepress/search', {
-      searchMaxSuggestions: 10,
-      test: null
-    }]
-  ]
-}

packages/@vuepress/theme-blog/layouts/Layout2.vue

-<template>
-  <div class="container">
-    <header>
-      <Content slot="header"/>
-    </header>
-    <main>
-      <Content/>
-    </main>
-    <footer>
-      <Content slot="footer"/>
-    </footer>
-  </div>
-</template>

packages/@vuepress/theme-blog/layouts/Tags.vue

-<template>
-  <div class="container">
-    <h2>Now is Tags Page</h2>
-  </div>
-</template>

packages/@vuepress/theme-blog/package.json

-{
-  "private": true,
-  "name": "@vuepress/theme-blog",
-  "version": "1.0.0-alpha.50",
-  "description": "theme-blog for vuepress",
-  "main": "index.js",
-  "publishConfig": {
-    "access": "public"
-  },
-  "repository": {
-    "type": "git",
-    "url": "git+https://github.com/vuejs/vuepress.git",
-    "directory": "packages/@vuepress/theme-blog"
-  },
-  "keywords": [
-    "documentation",
-    "vue",
-    "vuepress",
-    "generator"
-  ],
-  "dependencies": {
-    "@vuepress/plugin-blog": "^1.0.0-alpha.49",
-    "@vuepress/plugin-medium-zoom": "^1.0.0-alpha.50",
-    "@vuepress/plugin-pagination": "^1.0.0-alpha.50",
-    "@vuepress/plugin-search": "^1.0.0-alpha.50"
-  },
-  "author": "ULIVZ <chl814@foxmail.com>",
-  "license": "MIT",
-  "bugs": {
-    "url": "https://github.com/vuejs/vuepress/issues"
-  },
-  "homepage": "https://github.com/vuejs/vuepress/packages/@vuepress/theme-blog#readme"
-}

packages/blog/package.json

-{
-  "private": true,
-  "name": "blog",
-  "description": "blog of VuePress",
-  "version": "1.0.0-alpha.48",
-  "scripts": {
-    "dev": "vuepress dev source --temp .temp",
-    "build": "vuepress build source --temp .temp"
-  },
-  "dependencies": {
-    "@vuepress/theme-blog": "^1.0.0-alpha.48",
-    "vuepress": "^1.0.0-alpha.48"
-  }
-}

packages/blog/source/.vuepress/config.js

-module.exports = {
-  title: 'VuePress\'s Blog',
-  theme: '@vuepress/blog'
-}

packages/blog/source/_posts/1.html

-<template>
-  <div class="content"><h1 id="vuepress-blog"><a class="header-anchor" href="#vuepress-blog" aria-hidden="true">#</a>
-    VuePress' BLOG</h1>
-    <ul>
-      <li>Evan You</li>
-    </ul>
-    <template slot="projects">
-      <ul>
-        <li>Vue.js</li>
-        <li>VueX</li>
-      </ul>
-    </template>

packages/blog/source/_posts/1.md

----
-date: 2018-10-12
----
-
-# 1

packages/blog/source/index.md

-::: slot header
-# Here might be a page title
-:::
-
-- A Paragraph
-- Another Paragraph
-
-::: slot footer
-Here's some contact info
-:::
-