Don't purge *all* sync information when a user is deleted.

Bug: 132869699
Test: Manual:
1. Add two accounts
(Wait for a while)
2. Remove one
(Wait for a while)
3. Make sure the other account's sync adapter states are intact using
"dumpsys content".
4. Add a guest user
(Wait for a while)
5. Remove the guest user
(Wait for a while)
3. Make sure user-0's sync adapter states are intact using
"dumpsys content".

Change-Id: I1fc11a37d6b4672ecd4c62f338d75a7381a2df6b