Remove legacy L2 topology build option from UI

NetBrain currently allows users to build L2 topology using either the newer topology engine or a legacy build option, with the legacy option retained as a fallback path. Retaining this legacy option can lead to instability, including crash conditions, when it is selected instead of the current engine. This enhancement would remove the legacy L2 topology build option from the UI, leaving only the current topology engine as the supported build path. Simplifying this choice would reduce the risk of customers inadvertently selecting an outdated build method and would improve overall stability of L2 topology construction.