WindowManagerService: 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 WindowManagerService.

Bug: 208476087
Change-Id: Icec64be737709b63c14f0d2a73c9abf6894ce87e