graph_constructor: Remove dead code.

Optimizations such as constant-folding and common-subexpression-elimination
are applied directly on a Graph* (for example, by GraphOptimizer in
tensorflow/core/common_runtime/graph_optimizer.h).

I could not find any usage of the GraphConstructorOptions removed in this
change. Would like to keep this file focused on conversion from GraphDef to
Graph and leave operations on the in-memory representation elsewhere.
Change: 133447199