Remove `non-fused` version of `adjust_hue` as GPU kernel is already in place (#14187)

Was looking into adding batch support for `tf.image.random_hue` (8926)
and noticed that the `non-fused` version of `adjust_hue` was still
in place. The `non-fused` is for non-GPU support of `adjust_hue`.

As GPU kernel for `AdjustHue` has already been added in PR 6818,
I think it makes sense to remove the non-fused version. Besides,
the env `TF_ADJUST_HUE_FUSED` seems no in use anyway.

This fix removed `non-fused` version of `adjust_hue` as GPU kernel
is already in place.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>