Remove unreferenced footnotes which causes a build failure on ubuntu 18.04

diff -u -r ../llvm-6.0.0.src/docs/AMDGPUUsage.rst ./docs/AMDGPUUsage.rst
--- ../llvm-6.0.0.src/docs/AMDGPUUsage.rst	2017-12-12 06:47:00.000000000 +0100
+++ ./docs/AMDGPUUsage.rst	2018-04-30 00:58:07.492244676 +0200
@@ -4142,8 +4142,6 @@
 .. [AMD-GCN-GFX7] `AMD Sea Islands Series ISA <http://developer.amd.com/wordpress/media/2013/07/AMD_Sea_Islands_Instruction_Set_Architecture.pdf>`_
 .. [AMD-GCN-GFX8] `AMD GCN3 Instruction Set Architecture <http://amd-dev.wpengine.netdna-cdn.com/wordpress/media/2013/12/AMD_GCN3_Instruction_Set_Architecture_rev1.1.pdf>`__
 .. [AMD-GCN-GFX9] `AMD "Vega" Instruction Set Architecture <http://developer.amd.com/wordpress/media/2013/12/Vega_Shader_ISA_28July2017.pdf>`__
-.. [AMD-OpenCL_Programming-Guide]  `AMD Accelerated Parallel Processing OpenCL Programming Guide <http://developer.amd.com/download/AMD_Accelerated_Parallel_Processing_OpenCL_Programming_Guide.pdf>`_
-.. [AMD-APP-SDK] `AMD Accelerated Parallel Processing APP SDK Documentation <http://developer.amd.com/tools/heterogeneous-computing/amd-accelerated-parallel-processing-app-sdk/documentation/>`__
 .. [AMD-ROCm] `ROCm: Open Platform for Development, Discovery and Education Around GPU Computing <http://gpuopen.com/compute-product/rocm/>`__
 .. [AMD-ROCm-github] `ROCm github <http://github.com/RadeonOpenCompute>`__
 .. [HSA] `Heterogeneous System Architecture (HSA) Foundation <http://www.hsafoundation.com/>`__
@@ -4152,4 +4150,3 @@
 .. [YAML] `YAML Ain't Markup Language (YAML™) Version 1.2 <http://www.yaml.org/spec/1.2/spec.html>`__
 .. [OpenCL] `The OpenCL Specification Version 2.0 <http://www.khronos.org/registry/cl/specs/opencl-2.0.pdf>`__
 .. [HRF] `Heterogeneous-race-free Memory Models <http://benedictgaster.org/wp-content/uploads/2014/01/asplos269-FINAL.pdf>`__
-.. [AMD-AMDGPU-Compute-Application-Binary-Interface] `AMDGPU Compute Application Binary Interface <https://github.com/RadeonOpenCompute/ROCm-ComputeABI-Doc/blob/master/AMDGPU-ABI.md>`__