Revert "Add the ability to write shell files to the untrusted_app domain."

At this point, we still don't understand the root cause of
bug 10290009, or if it's even a real bug.  Rollback
29d0d406 so we an get a device
in this state and figure out the root cause of this problem.

This reverts commit 29d0d406.

Bug: 10290009