Remove redundant test
Remove redundant test

- We don't need the `ArgumentError` behaviour test any more, as recent
  versions of `gds-api-adapters` handle the error as part of the test
  stub. GDS API Adapter is one of our gems, it's well testetd within
  itself - there's no point us asserting that we get _its_ errors, I
  don't think. We were seeing:

```
  1) Failure:
TransitionConfigSiteTest#test_site_create_fails_when_no_slug [/Users/issylong/govuk/transition-config/tests/lib/transition-config/site_test.rb:139]:
[ArgumentError] exception expected, not
Class: <GdsApi::HTTPNotFound>
Message: <"URL: https://www.gov.uk/api/organisations/non-existent-whitehall-slug\nResponse body:\n\n\nRequest body:\n">
---Backtrace---
/Users/issylong/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/gds-api-adapters-40.0.0/lib/gds_api/json_client.rb:145:in `rescue in do_json_request'
/Users/issylong/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/gds-api-adapters-40.0.0/lib/gds_api/json_client.rb:136:in `do_json_request'
/Users/issylong/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/gds-api-adapters-40.0.0/lib/gds_api/json_client.rb:80:in `get_json'
/Users/issylong/.rbenv/versions/2.6.3/lib/ruby/2.6.0/forwardable.rb:230:in `get_json'
/Users/issylong/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/gds-api-adapters-40.0.0/lib/gds_api/organisations.rb:9:in `organisation'
/Users/issylong/govuk/transition-config/lib/transition-config/organisations.rb:21:in `find'
/Users/issylong/govuk/transition-config/lib/transition-config/site.rb:190:in `create'
/Users/issylong/govuk/transition-config/tests/lib/transition-config/site_test.rb:140:in `block in test_site_create_fails_when_no_slug'
---------------
```

alphagov/transition-config,alphagov/transition-config