Revert "fix: allow an object's Symbols to be observed (vuejs#6704)"

This reverts commit 4fd2ce8.

src/shared/util.js

@@ -67,7 +67,7 @@ export function isRegExp (v: any): boolean {
  * Check if val is a valid array index.
  */
 export function isValidArrayIndex (val: any): boolean {
-  const n = parseFloat(String(val))
+  const n = parseFloat(val)
   return n >= 0 && Math.floor(n) === n && isFinite(val)
 }