drm/i915/gt/uc: drop unused but set variable sseu

Prepare for re-enabling -Wunused-but-set-variable.

Apparently sseu is leftover from commit 9a92732f ("drm/i915/gt: Add
general DSS steering iterator to intel_gt_mcr").

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/d542f25bffd5a50ff621bee93415a972c7768a2a.1685119007.git.jani.nikula@intel.com