remove walk-sync

It appears to be unused

Author: Kelly Selden

package.json

@@ -69,8 +69,7 @@
     "mocha": "^2.2.4",
     "request": "^2.55.0",
     "sinon": "^1.17.4",
-    "symlink-or-copy": "^1.0.1",
-    "walk-sync": "^0.2.5"
+    "symlink-or-copy": "^1.0.1"
   },
   "engines": {
     "node": ">= 4.0.0"