Remove deprecated file system APIs

Towards #8751.

Removes APIs listed in #6127. As you can see in that issue, there are a number of extensions that access it, but it's nearly a year since they were deprecated. It's time to remove them.
This was [a part](https://github.com/adobe/brackets/pull/9490#discussion_r18744298) of the auto-install extensions PR, but was reverted during review. Since I was editing this file, I thought I would go ahead and restore it.