Code health - Remove duplicate code

[parthea]

The jinja templates here and here have a lot of code duplication. When there is a bug, a fix is being applied in more than one place. For example see https://github.com/googleapis/gapic-generator-python/pull/1582/files where a fix was applied in 5 places instead of one. This breaks the DRY (Don't repeat yourself) rule of code health

[parthea]

Transferring to google-cloud-python as the code for gapic-generator will be migrated there.