Remove option `add-iam-methods`

[parthea]

The add-iam-methods option and corresponding nox session showcase_unit_add_iam_methods should be removed from the codebase as it is redundant code.

https://github.com/googleapis/gapic-generator-python/blob/515dc8ac17b9a8177feaa84416ab88e671c5359c/gapic/utils/options.py#L57

https://github.com/googleapis/gapic-generator-python/blob/515dc8ac17b9a8177feaa84416ab88e671c5359c/gapic/templates/%25namespace/%25name_%25version/%25sub/services/%25service/client.py.j2#L627

https://github.com/googleapis/gapic-generator-python/blob/515dc8ac17b9a8177feaa84416ab88e671c5359c/noxfile.py#L513

Before it can be removed, we need to remove usage of it here: https://github.com/search?q=repo%3Agoogleapis%2Fgoogleapis%20add-iam-methods&type=code

We shouldn't need the add-iam-methods as mentioned in https://github.com/googleapis/gapic-generator-python/pull/1240