python: make autogen-snippets options consistent

[jskeet]

Comment on googleapis/google-cloud-python#16762

Unrelated to the changes in this PR, in this file we have autogen-snippets=False and autogen-snippets without any True/False value. It seems that we should either have autogen-snippets=False, or not specify autogen-snippets at all.

These values were copied from BUILD.bazel as part of migration, but we should tidy them up.