Update SyncStorageEngine#DELETE_LEGACY_PARCEL_FILES.

Delete the parcel files on a successful upgrade to proto. This was
kept as true such that a revert to the parcel files would be possible
if needed.

Bug: 143656271
Test: manual: files are deleted on upgrade.
Change-Id: I98019e2aef3e4413a67c714691ebdc0973a8a665