Fix JS caching bug with multiple image uploads on same page

I found a bug:

Steps to reproduce:
1. Create a settings page with 2 image fields.
2. View that settings page
3. Browse/upload/select image field #1.
4. Browse/upload/select image field #2.

Expected:
Image #1 in #1 slot, Image #2 in #2 slot

What actually happens:
Image #2 replaces the #1 slot. Essentially the uploader remembers the first field you edited, then all subsequent edits (no matter which button you click on) updates the first field you edited.

It appears the JS object "custom_uploader" remembers the field. I'm assuming this code was in place for performance/optimization reasons, but removing it does seem to resolve this bug.