drm/sched: Delete unused update_job_credits

No driver is using the update_job_credits() schduler vfunc
so lets remove it.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Danilo Krummrich <dakr@redhat.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Philipp Stanner <pstanner@redhat.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Acked-by: Boris Brezillon <boris.brezillon@collabora.com>
Acked-by: Matt Coster <matt.coster@imgtec.com>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250110111301.76909-1-tvrtko.ursulin@igalia.com