drm/i915/guc: Drop leftover preemption code

Remove unused enums and ctx_save_restore_disabled() function, leftover
from the legacy preemption removal.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191205220243.27403-1-daniele.ceraolospurio@intel.com