drm/xe/uapi: Remove support for persistent exec_queues

Persistent exec_queues delays explicit destruction of exec_queues
until they are done executing, but destruction on process exit
is still immediate. It turns out no UMD is relying on this
functionality, so remove it. If there turns out to be a use-case
in the future, let's re-add.

Persistent exec_queues were never used for LR VMs

v2:
- Don't add an "UNUSED" define for the missing property
  (Lucas, Rodrigo)
v3:
- Remove the remaining struct xe_exec_queue::persistent state
  (Niranjana, Lucas)

Fixes: dd08ebf6

 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Francois Dugast <francois.dugast@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Acked-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240209113444.8396-1-thomas.hellstrom@linux.intel.com