message: Remove environment variable to disable C API.
  This is staging for removing the _USE_C_API toggle altogether.
diff: @@ -59,11 +59,9 @@ from tensorflow.python.util import tf_contextlib
  from tensorflow.python.util.tf_export import tf_export
  -# Temporary global switch determining if we should enable the work-in-progress
  -# calls to the C API. Currently disabled by default but can be manually enabled
  -# in code or via the environment variable. This will be removed once all
  -# functionality is supported and there's no performance penalty with it enabled.
  -_USE_C_API = os.getenv("TF_C_API_GRAPH_CONSTRUCTION", "1") is not "0"
  +# Temporary global switches determining if we should enable the work-in-progress
  +# calls to the C API. These will be removed once all functionality is supported.
  +_USE_C_API = True
  _USE_C_SHAPES = os.getenv("TF_C_API_GRAPH_CONSTRUCTION_SHAPES", "0") is not "0"
  