graph: remove obsolete comments

Graph.Get function already returns nil when the image doesn't exist so
following comment is obsolete.
FIXME: return nil when the image doesn't exist, instead of an error

Signed-off-by: Daehyeok.Mun <daehyeok@gmail.com>
Merge pull request #7782 from somaopensource/graph_remove_fixme

graph: remove obsolete comments