Remove SliceD methods

[bernardnormier]

We should look into removing SliceD methods, like #3737 removes ProtobufD methods.

For example:

   [SliceOperation("opWithMultipleParametersAndReturnValues")]
    [global::System.ComponentModel.EditorBrowsable(global::System.ComponentModel.EditorBrowsableState.Never)]
    protected static async global::System.Threading.Tasks.ValueTask<IceRpc.OutgoingResponse> SliceDOpWithMultipleParametersAndReturnValuesAsync(
        IMyOperationsAService target,
        IceRpc.IncomingRequest request,
        global::System.Threading.CancellationToken cancellationToken)
    {
        request.CheckNonIdempotent();
        var args = await Request.DecodeOpWithMultipleParametersAndReturnValuesAsync(request, cancellationToken).ConfigureAwait(false);
        var returnValue = await target.OpWithMultipleParametersAndReturnValuesAsync(args.P1, args.P2, request.Features, cancellationToken).ConfigureAwait(false);
        return new IceRpc.OutgoingResponse(request)
        {
            Payload = Response.EncodeOpWithMultipleParametersAndReturnValues(returnValue.R1, returnValue.R2, request.Features.Get<ISliceFeature>()?.EncodeOptions),
            PayloadContinuation = null
        };
    }

The idea is for the source generator to generate equivalent code in the DispatchAsync "switch/case". We'd generate Request.DecodeXxx and Response.EncodeXxx with slice-cs as usual.

If needed, we could add additional information in the SliceOperationAttribute.