Remove USM extension

This was also adopted into SYCL 2020.

Author: gmlueck

sycl/doc/extensions/README.md

@@ -24,7 +24,6 @@ DPC++ extensions status:
 | [SYCL_INTEL_static_local_memory_query](StaticLocalMemoryQuery/SYCL_INTEL_static_local_memory_query.asciidoc)                | Proposal                                  | |
 | [Sub-groups for NDRange Parallelism](SubGroupNDRange/SubGroupNDRange.md)                                                    | Deprecated(OpenCL: CPU, GPU)              | |
 | [Sub-groups](SubGroup/SYCL_INTEL_sub_group.asciidoc)                                                                        | Deprecated                                | |
-| [Unified Shared Memory](USM/USM.adoc)                                                                                       | Supported(OpenCL)                         | |
 | [Use Pinned Memory Property](UsePinnedMemoryProperty/UsePinnedMemoryPropery.adoc)                                           | Supported                                 | |
 | [Level-Zero backend specification](LevelZeroBackend/LevelZeroBackend.md)                                           	      | Supported                                 | |
 | [Platform Context](PlatformContext/PlatformContext.adoc) | Proposal | |