PackageManagerService: remove unused support for onlyCore mode

Due to the removal of support for Full Disk Encryption,
the "core apps only" mode of system_server is no longer used, and
onlyCore is hard-coded to false.  For details, see http://ag/17685636.

Remove the corresponding obsolete code from PackageManagerService and
the related classes InitAppsHelper, UserDataPreparer, PackageParser2,
and ParsingPackageUtils.

Bug: 208476087
Change-Id: I48289d1f74414f831504b38ac5c1bde719b07fb2