R1:9a44a81e797d

Remove unused save() method in PhabricatorUserCache

Summary: Seems to be unused. Plus it calls `Filesystem::digestForIndex()` which does not exist. So if this method was ever called then it would through an exception and not reach the `parent::save()` call either in the line after.

Test Plan:
Grep and read the codebase.

Tried things that may use cache and they still work, including:

- running `./bin/cache purge --caches user` successfully shows `Purging "user" cache...`
- the button "Purge cache" in…