Fix ts-node for Windows (#2101)

Summary:
Somehow referring to the parent directory doesn't work on Windows. Please don't ask me why. Given that we run a script from outside the package doing it on the top-level seems more consistent anyway.

Pull Request resolved: https://github.com/facebook/flipper/pull/2101

Test Plan: Yarn on Windows succeeds: https://github.com/facebook/flipper/pull/2101/checks?check_run_id=2195613718

Reviewed By: fabiomassimo

Differential Revision: D27335250

Pulled By: passy

fbshipit-source-id: aaccdb3f7e78ca264b371a7723c9bad065919d45

desktop/types/package.json

@@ -10,8 +10,5 @@
   "keywords": [
     "Flipper"
   ],
-  "author": "Facebook, Inc",
-  "scripts": {
-    "postinstall": "../ts-node ../scripts/gen-type-index.ts"
-  }
+  "author": "Facebook, Inc"
 }