CmsBlock SSR fix with new serverPrefetch method

[maikel-code]

To make SSR finally work with cmsBlock:

• just rename asyncData to serverPrefetch.
• remove function params (as not needed) and outdated comments
• change store.dispatch to this.$store.dispatch

And BAM, just some new vue magic and it works as intended
serverPrefetch is out there since vue version 2.6 and gives you access to this object
https://ssr.vuejs.org/api/#serverprefetch

serverPrefetch () {
    return new Promise((resolve, reject) => {
      this.$store.dispatch('cmsBlock/single', {
        value: this.identifier
      }).then(res => {
        return resolve(res)
      })
    })
  }