Removes a redundant canary test

src/ruby/spec/credentials_spec.rb

@@ -68,10 +68,4 @@ describe Credentials do
       expect { cred1.compose(cred2) }.to_not raise_error
     end
   end
-
-  describe 'Credentials#default' do
-    it 'is not implemented yet' do
-      expect { Credentials.default }.to raise_error RuntimeError
-    end
-  end
 end