Have only a single definition of standard_coders.yaml

There are two copies of standard_coders.yaml defined:

• https://github.com/apache/beam/blob/master/model/fn-execution/src/main/resources/org/apache/beam/model/fnexecution/v1/standard_coders.yaml
• https://github.com/apache/beam/blob/master/sdks/python/apache_beam/testing/data/standard_coders.yaml

The Python SDK specific instance should be removed.