drm/i915/ttm: remove unused function

intel_region_ttm_node_free is no longer used. Also fixup the related
kerneldoc.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210617083719.497619-1-matthew.auld@intel.com