IVGCVSW-1985: Replace std::string* reasonIfNotSupported with Optional<std::string&> in IsLayerSupported implementations

Change-Id: I2f054f0dcff9bdc86ee90c55b3e94c6b4ae25085
diff --git a/src/backends/cl/workloads/ClConvertFp32ToFp16Workload.hpp b/src/backends/cl/workloads/ClConvertFp32ToFp16Workload.hpp
index 95d1990..28d0bfa 100644
--- a/src/backends/cl/workloads/ClConvertFp32ToFp16Workload.hpp
+++ b/src/backends/cl/workloads/ClConvertFp32ToFp16Workload.hpp
@@ -23,8 +23,6 @@
     mutable arm_compute::CLDepthConvertLayer m_Layer;
 };
 
-arm_compute::Status ClConvertFp32ToFp16WorkloadValidate(const TensorInfo& input,
-                                                        const TensorInfo& output,
-                                                        std::string* reasonIfUnsupported);
+arm_compute::Status ClConvertFp32ToFp16WorkloadValidate(const TensorInfo& input, const TensorInfo& output);
 
 } //namespace armnn