Remove redundant check to prevent installing twice

It turns out Vue now does this check for all plugins
automatically, as pointed out by @thgh:
670a1d5#commitcomment-19918130

Author: foxbenjaminfox

src/index.js

@@ -1,14 +1,9 @@
 const prefix = '_async_computed$'
 
-let installed = false
-
 const AsyncComputed = {
   install (Vue, options) {
     options = options || {}
 
-    if (installed) return
-    installed = true
-
     Vue.config
       .optionMergeStrategies
       .asyncComputed = Vue.config.optionMergeStrategies.computed