Remove graph_callable.py and all references to it.

Our APIs for creating functions are tfe.defun() and tfe.make_template();
graph_callable is no longer needed.

Additionally, change GraphModeFunction to accept a FuncGraph instead of
a CapturingGraph --- graph_callable was preventing us from doing this.

This is part of ongoing hygiene work to help us think more clearly about the function APIs we'll want to provide. GraphModeFunction (which is renamed to GraphCallable here) previously had way too many attributes.

PiperOrigin-RevId: 209502276