Removed a redundant C++ unit test wrapper

Removed test script `test_normal_flux_condition.py`, which executed two
C++ unit tests: `TestCalculateHorizontalNormalFlux` and
`TestCalculateInclinedNormalFlux`. These unit tests, however, are also
executed through test script `run_cpp_tests.py`, since both are part of
test suite `KratosGeoMechanicsFastSuite`. As such, the former test
script was redundant and it has been removed now.

A few other changes were carried out to address three code smells
reported by SonarQube and some problems detected by Clang-tidy.

applications/GeoMechanicsApplication/tests/cpp_tests/test_Pw_normal_flux_condition.cpp

@@ -11,18 +11,14 @@
 //
 
 #include <string>
-#include <iostream>
 
 // Project includes
 #include "containers/model.h"
 #include "testing/testing.h"
 #include "custom_conditions/Pw_normal_flux_condition.hpp"
 
-namespace Kratos
+namespace Kratos::Testing
 {
-    namespace Testing
-    {
-
         KRATOS_TEST_CASE_IN_SUITE(CalculateHorizontalNormalFlux, KratosGeoMechanicsFastSuite)
         {
 
@@ -58,7 +54,6 @@ namespace Kratos
 
             // Create the test piping element
             std::vector<ModelPart::IndexType> cond_nodes{1, 2};
-            // auto p_element = r_model_part.CreateNewElement("SteadyStatePwPipingElement2D4N", 1, element_nodes, p_elem_prop);
             auto cond = r_model_part.CreateNewCondition("PwNormalFluxCondition2D2N", 1, cond_nodes, cond_prop);
             // Initialize the element to initialize the constitutive law
             const auto &r_process_info = r_model_part.GetProcessInfo();
@@ -122,7 +117,6 @@ namespace Kratos
 
             // Create the test piping element
             std::vector<ModelPart::IndexType> cond_nodes{1, 2};
-            // auto p_element = r_model_part.CreateNewElement("SteadyStatePwPipingElement2D4N", 1, element_nodes, p_elem_prop);
             auto cond = r_model_part.CreateNewCondition("PwNormalFluxCondition2D2N", 1, cond_nodes, cond_prop);
             // Initialize the element to initialize the constitutive law
             const auto &r_process_info = r_model_part.GetProcessInfo();
@@ -150,6 +144,4 @@ namespace Kratos
                     1.0e-6);
             }
         }
-
-    }
 }