Don't call `setRootWorkflow` when deleting workflow

In src/edition/Sidebar/sidebarMenu/WorkflowSidebarMenu.tsx:

setRootWorkflow(EMPTY_GRAPH, rfInstance, tasks);

This is not needed since we clear the workflow query param, which re-renders the Canvas with an undefined workflow ID, which leads to the above being called in the onInit callback of the ReactFlow component.