drm/xe: Nuke EXEC_QUEUE_FLAG_PERSISTENT

This is a left over of commit f1a9abc0 ("drm/xe/uapi: Remove support for persistent exec_queues").

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240307135229.41973-3-jose.souza@intel.com