IVGCVSW-4483 Remove boost::polymorphic_downcast

 * exchange boost::polymorphic_downcast with armnn::PolymorphicDowncast
 * remove unnecessary includes of boost::polymorphic_downcast

Signed-off-by: Jan Eilers <jan.eilers@arm.com>
Change-Id: Ie603fb82860fe05fee547dc78073230cc62b2e1f
diff --git a/src/backends/cl/ClBackendContext.cpp b/src/backends/cl/ClBackendContext.cpp
index f612c37..bfe93bd 100644
--- a/src/backends/cl/ClBackendContext.cpp
+++ b/src/backends/cl/ClBackendContext.cpp
@@ -8,14 +8,13 @@
 
 #include <armnn/Logging.hpp>
 #include <armnn/utility/Assert.hpp>
+#include <armnn/utility/PolymorphicDowncast.hpp>
 
 #include <arm_compute/core/CL/OpenCL.h>
 #include <arm_compute/core/CL/CLKernelLibrary.h>
 #include <arm_compute/runtime/CL/CLScheduler.h>
 #include <arm_compute/runtime/CL/CLTunerTypes.h>
 
-#include <boost/polymorphic_cast.hpp>
-
 namespace armnn
 {
 
@@ -161,7 +160,7 @@
     bool useLegacyTunerAPI = options.m_GpuAccTunedParameters.get() != nullptr;
     if (useLegacyTunerAPI)
     {
-        auto clTunerParams = boost::polymorphic_downcast<ClTunedParameters*>(
+        auto clTunerParams = PolymorphicDowncast<ClTunedParameters*>(
                                 options.m_GpuAccTunedParameters.get());
         tuner = &clTunerParams->m_Tuner;