chore($plugin-blog): do not fallback layout in plugin

[meteorlxy]

Summary

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

Other information:

Two reasons for this PR:

• It's not reasonable to fallback the layout in the frontmatter to Page. Page usually means those layouts with <Content/> component, but the Tags and Categories are extra pages that added in the plugin-blog with not content.
• We've already fallback layout in LayoutDistributor:
  

  vuepress/packages/@vuepress/core/lib/app/components/LayoutDistributor.vue

  Lines 15 to 19 in 6146287

  	if (this.$page.path) {
  	if (getLayoutAsyncComponent(this.$page.frontmatter.layout)) {
  	return this.$page.frontmatter.layout
  	}
  	return 'Layout'