DisplayManagerService: remove unused support for onlyCore mode

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 DisplayManagerService.

Bug: 208476087
Change-Id: If53532e1404048d644f6f805a6882e034e914fa9