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