Replace tf2xla/dump_graph.h with core/util/dump_graph.h.

This changes the location of the environment variable to enable dumping in the bridge from "TF_XLA_FLAGS=--tf_dump_graph_prefix=/tmp/" to "TF_DUMP_GRAPH_PREFIX=/tmp/".  This then allows both Tensorflow and TF2XLA bridge dumping to use the same environment variable to specify the path, rather than having 2 different flags with the same intent.

PiperOrigin-RevId: 236829860