IVGCVSW-1964 : replace optional biases with home-grown Optional

!android-nn-driver:151788

Change-Id: Ibdc41d09b8df05e7a0360dcb8a060860dfb1bd99
diff --git a/src/backends/cl/workloads/ClConvolution2dWorkload.hpp b/src/backends/cl/workloads/ClConvolution2dWorkload.hpp
index 14a39f3..a5de876 100644
--- a/src/backends/cl/workloads/ClConvolution2dWorkload.hpp
+++ b/src/backends/cl/workloads/ClConvolution2dWorkload.hpp
@@ -13,8 +13,6 @@
 #include <arm_compute/runtime/CL/functions/CLConvolutionLayer.h>
 #include <arm_compute/runtime/MemoryManagerOnDemand.h>
 
-#include <boost/optional.hpp>
-
 #include <memory>
 
 namespace armnn
@@ -24,7 +22,7 @@
                                                     const TensorInfo& output,
                                                     const Convolution2dDescriptor& descriptor,
                                                     const TensorInfo& weights,
-                                                    const boost::optional<TensorInfo>& biases);
+                                                    const Optional<TensorInfo>& biases);
 
 class ClConvolution2dWorkload : public BaseWorkload<Convolution2dQueueDescriptor>
 {