remove unnecessary method from base 64 serializer

lib/serializers/base-64.js

@@ -40,19 +40,6 @@ function deserialize(string) {
   return state
 }
 
-/**
- * Deserialize a Document `string`.
- *
- * @param {String} string
- * @return {Document} document
- */
-
-function deserializeDocument(string) {
-  const raw = decode(string)
-  const state = Raw.deserialize(raw)
-  return state.document
-}
-
 /**
  * Deserialize a Node `string`.
  *
@@ -98,7 +85,6 @@ function serializeNode(node) {
 
 export default {
   deserialize,
-  deserializeDocument,
   deserializeNode,
   serialize,
   serializeNode