API Core: Simplify from_rfc3339 methods

[crwilcox]

from_rfc3339 doesn't handle Nanos, but from_rfc3339_nanos does. Would it make sense to use the from_rfc3339_nanos implementation for both? It seems to me we could simplify the interface by having one from rfc3339 method.

https://github.com/googleapis/google-cloud-python/blob/master/api_core/google/api_core/datetime_helpers.py#L117

https://github.com/googleapis/google-cloud-python/blob/master/api_core/google/api_core/datetime_helpers.py#L130