Remove impersonation lifetime.

Per dbt-labs#769 , we shouldn't
need to set the lifetime of an impersonated token. In some edge cases,
attempting to set it to the job execution timeout (> 300 seconds) can
result in authentication failures, even though the job will complete
successfully and authentication creds will be correctly refreshed.

dbt/adapters/bigquery/connections.py

@@ -348,7 +348,6 @@ def get_impersonated_credentials(cls, profile_credentials):
             source_credentials=source_credentials,
             target_principal=profile_credentials.impersonate_service_account,
             target_scopes=list(profile_credentials.scopes),
-            lifetime=(profile_credentials.job_execution_timeout_seconds or 300),
         )
 
     @classmethod