refactor(reactivity): remove stale API markReadonly

BREAKING CHANGE: `markReadonly` has been removed.

packages/reactivity/src/index.ts

@@ -7,7 +7,6 @@ export {
   isReadonly,
   shallowReadonly,
   toRaw,
-  markReadonly,
   markNonReactive
 } from './reactive'
 export {

packages/reactivity/src/reactive.ts

@@ -20,7 +20,6 @@ const readonlyToRaw = new WeakMap<any, any>()
 
 // WeakSets for values that are marked readonly or non-reactive during
 // observable creation.
-const readonlyValues = new WeakSet<any>()
 const nonReactiveValues = new WeakSet<any>()
 
 const collectionTypes = new Set<Function>([Set, Map, WeakMap, WeakSet])
@@ -47,10 +46,6 @@ export function reactive(target: object) {
   if (readonlyToRaw.has(target)) {
     return target
   }
-  // target is explicitly marked as readonly by user
-  if (readonlyValues.has(target)) {
-    return readonly(target)
-  }
   if (isRef(target)) {
     return target
   }
@@ -156,11 +151,6 @@ export function toRaw<T>(observed: T): T {
   return reactiveToRaw.get(observed) || readonlyToRaw.get(observed) || observed
 }
 
-export function markReadonly<T>(value: T): T {
-  readonlyValues.add(value)
-  return value
-}
-
 export function markNonReactive<T extends object>(value: T): T {
   nonReactiveValues.add(value)
   return value