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 /config/cache/ still work.
- editing a user
- changing user picture using default one
- uploading new user picture
- creating Calendar event and inviting yourself
- visiting user settings
- changing a user setting, including Timezone, Display preferences

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25645

src/applications/people/storage/PhabricatorUserCache.php

@@ -32,11 +32,6 @@ final class PhabricatorUserCache extends PhabricatorUserDAO {
     ) + parent::getConfiguration();
   }
 
-  public function save() {
-    $this->cacheIndex = Filesystem::digestForIndex($this->getCacheKey());
-    return parent::save();
-  }
-
   public static function writeCache(
     PhabricatorUserCacheType $type,
     $key,