Remove obsolete TODO in LockSettingsStorage

The referenced bug is already closed as fixed.  And secdiscard does not
really work anyway.  But even if it did, it would be redundant to do it
on a user's synthetic password files when that user is being removed,
since those files are located in the user's DE storage.

Bug: 34600579
Test: N/A
Flag: EXEMPT comment-only change
Change-Id: Ia473d88a6a819907e7ab24a9cc21ee7264849a73