[SavedModelBundleLite] Avoid copying the GraphDef during the load path.
[SavedModelBundleLite] Avoid copying the GraphDef during the load path.

Previously, we reused the "legacy" SavedModelBundle loading path, then discarded the unused MetaGraphDef. An old TODO called out the opportunity to avoid copying the MetaGraphDef's GraphDef, and this change does that.
