Update `tf.nn.in_top_k` to use v2 kernel.

This fix is a follow up to PR #11197 so that `tf.nn.in_top_k`
uses V2 kernel now.

This fix follows the API compatibility workflow (3 weeks).

This fix also updates related tests so that `gen_nn_ops` could
be removed.

This fix is related to #9717.

NOTE: In the original PR #11197, the `_in_top_kv2` was incorrectly
called even the comments specified to wait for 3 weeks. That
was fixed by commit https://github.com/tensorflow/tensorflow/commit/3e3eebd6adb3512b69e28b6e7acce9a59527f699


(Thanks @vrv! for the fix).

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