[SYCL][DOC] Move discard queue events to supported (#5452)

This extension was recently implemented.  Move its specification to
the "supported" directory.

Author: gmlueck

sycl/doc/extensions/README.md

@@ -12,7 +12,6 @@ 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)              | |
 | [Property List](PropertyList/SYCL_EXT_ONEAPI_property_list.asciidoc)                                                        | Proposal                                  | |
-| [DiscardQueueEvents](DiscardQueueEvents/SYCL_EXT_ONEAPI_DISCARD_QUEUE_EVENTS.asciidoc) | Proposal | |
 
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