Removed DPM.factoryReset() method used by ManagedProvisioning app.

It's redundant, as DPM.wipeData() do the same (and is now vibible
to apps holding the MASTER_CLEAR permission).

Test: manual verification trying to install a DPC that doesn't exist

Bug: 175392542
Bug: 171603586
Bug: 172697975

Change-Id: I0ed88a6eb239239c38cf569bf21c0745518925e4