fix: Nightly dev test dur to updates in 'wait_for_process_finished'. (#4575)

## Context
The test fixture was already exiting the solver session, thus exit was
called twice.

## Change Summary
Removed exit call from the test.

## Rationale
As the exit was already called in the fixture.


This updated behavior is correct as redundant call to exit will be
caught now.

Author: prmukherj

doc/changelog.d/4575.fixed.md

@@ -0,0 +1 @@
+Nightly dev test due to updates in 'wait_for_process_finished'.

tests/integration/test_optislang/test_optislang_integration.py

@@ -140,8 +140,6 @@ def test_simple_solve(mixing_elbow_param_case_data_session):
     # A bug has been submitted to address this
     assert output_unit == "K"
 
-    solver_session.exit()
-
 
 @pytest.mark.nightly
 @pytest.mark.codegen_required