Remove excess check_response call in std_dev_for_parameter

The `std_dev_for_parameter` method manually checks if the response's status_code is
200 OK and decides what to do if it is not. Calling the `check_response`
method raises an exception if it is not 200 OK, which is a valid
response in this case.

src/ert/gui/tools/plot/plot_api.py

@@ -265,7 +265,6 @@ def std_dev_for_parameter(
                 params={"z": z},
                 timeout=self._timeout,
             )
-            self._check_response(response)
 
             if response.status_code == 200:
                 # Deserialize the numpy array