[SYCL] Remove doc [[intel::reqd_work_group_size]] (#5387)

This extension document describes an old form of the C++ attribute
`[[intel::reqd_work_group_size]]`, which is in the `intel` namespace.
This attribute has been adopted into SYCL 2020 in the `sycl` namespace,
and the compiler does not even recognize the form in the `intel`
namespace.  There is, therefore, no need to keep this old extension
document.

Author: gmlueck

sycl/doc/extensions/README.md

@@ -24,7 +24,6 @@ DPC++ extensions status:
 | [Queue Shortcuts](QueueShortcuts/QueueShortcuts.adoc)                                                                       | Supported                                 | |
 | [Reductions for ND-Range Parallelism](Reduction/Reduction.md)                                                               | Partially supported(OpenCL: CPU, GPU; CUDA) | Not supported: multiple reduction vars, multi-dimensional reduction vars |
 | [SYCL_INTEL_relax_standard_layout](RelaxStdLayout/SYCL_INTEL_relax_standard_layout.asciidoc)                                | Supported                                 | |
-| [SYCL_INTEL_reqd_work_group_size](ReqdWorkGroupSize/SYCL_INTEL_reqd_work_group_size.asciidoc)                               | Supported(OpenCL: CPU, GPU)               | |
 | [SPV_INTEL_function_pointers](SPIRV/SPV_INTEL_function_pointers.asciidoc)                                                   | Supported(OpenCL: CPU, GPU; HOST)         | |
 | [SPV_INTEL_inline_assembly](SPIRV/SPV_INTEL_inline_assembly.asciidoc)                                                       | Supported(OpenCL: GPU)                    | |
 | [LocalMemory](LocalMemory/LocalMemory.asciidoc)                                                                             | Supported(OpenCL; CUDA)                   | Revision 1 of the spec is fully supported, future revisions are expected to expand the functionality |