drm/i915/guc: Delete GuC code unused in future patches

Delete GuC code unused in future patches that rewrite the GuC interface
to work with the new firmware. Most of the code deleted relates to
workqueues or execlist port. The code is safe to remove because we still
don't allow GuC submission to be enabled, even when overriding the
modparam, so it currently can't be reached.

The defines + structs for the process descriptor and workqueue remain.
Although the new GuC interface does not require either of these for the
normal submission path multi-lrc submission does. The usage of the
process descriptor and workqueue for multi-lrc will be quite different
from the code that is deleted in this patch. A future patch will
implement multi-lrc submission.

v2: add a code in the commit message about the code being safe to
remove (Chris)

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: John Harrison <john.c.harrison@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20210113021236.8164-2-daniele.ceraolospurio@intel.com