Remove redundant `static_assert`s
Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this [failed invocation](https://github.com/tensorflow/tensorflow/pull/60025/checks?check_run_id=12067066574) of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.
Removed redundant `static_assert`s. They seem to have been introduced in 0b6b491d21d6 with the switch from `tensorflow::int64` to `int64_t`.