Remove references to map_external_memory_buffer

- References to `map_external_memory_buffer` have been removed as they
were proposed features, but they are not yet in the experimental
implementation

Author: przemektmalon

sycl/doc/extensions/experimental/sycl_ext_oneapi_bindless_images.asciidoc

@@ -1218,27 +1218,13 @@ image_mem_handle map_external_memory_array(
     interop_mem_handle interopMemHandle,
     const image_descriptor &imageDescriptor,
     const sycl::queue &syclQueue);
-
-void *map_external_memory_buffer(
-    interop_mem_handle interopMemHandle,
-    const sycl::device &syclDevice,
-    const sycl::context &syclContext);
-void *map_external_memory_buffer(
-    interop_mem_handle interopMemHandle,
-    const sycl::queue &syclQueue);
 }
 ```
 
-The resulting `interop_mem_handle` can then be mapped to two different memory 
-types, a buffer, or an array.
-
-If mapped to a buffer, the resulting `void*` can be used in USM like fashion. 
-It can be passed as an argument to a kernel and therein manipulated.
-
-If mapped to an array, the resulting type is an `image_mem_handle`, which can be
-used to construct images in the same way as memory allocated through 
-`alloc_image_mem`. The `ext_oneapi_copy` operations also work with imported 
-memory mapped to `image_mem_handle` types.
+The resulting `interop_mem_handle` can then be mapped, where the resulting type 
+is an `image_mem_handle`. This can be used to construct images in the same way 
+as memory allocated through `alloc_image_mem`. The `ext_oneapi_copy` operations 
+also work with imported memory mapped to `image_mem_handle` types.
 
 When calling `create_image` with an `image_mem_handle` mapped from an external 
 memory object, the user must ensure that the image descriptor they pass to