drm/i915: Remove deadcode

i915_active_acquire_for_context() was added in 2020 by
commit 5d934137 ("drm/i915: Export a preallocate variant of
i915_active_acquire()") but has never been used.

The last use of __i915_gem_object_is_lmem() was removed in 2021 by
commit ff20afc4 ("drm/i915: Update error capture code to avoid using
the current vma state")

Remove them.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Tvrtko Ursulin <tursulin@igalia.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241227113754.25871-1-tursulin@igalia.com