remove old prevState logic from transforms

src/models/state.js

@@ -58,7 +58,7 @@ class State extends new Record(DEFAULTS) {
 
     const state = new State({ document, selection })
 
-    return state.transform({ normalized: false })
+    return state.transform()
       .normalize(SCHEMA)
       .apply({ save: false })
   }

src/models/transform.js

@@ -29,7 +29,6 @@ class Transform {
   constructor(properties) {
     const { state, normalized = true } = properties
     this.state = state
-    this.prevState = normalized ? state : null
     this.operations = []
   }