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.

gapic-generator-python/gapic/utils/options.py

Line 57 in 515dc8a

'add-iam-methods', # microgenerator implementation for `reroute_to_grpc_interface`

gapic-generator-python/gapic/templates/%namespace/%name_%version/%sub/services/%service/client.py.j2

Line 627 in 515dc8a

{% if opts.add_iam_methods %}

gapic-generator-python/noxfile.py

Line 513 in 515dc8a

with showcase_library(session, other_opts=("add-iam-methods",)) as lib:

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 #1240