The now redundant "X" checkbox is removed since it's also present in the Symmetry panel above. The Topology Mirror is moved into the Symmetry panel also. This was needed because `Mesh.use_x_mirror` has recently been turned into different functionality, and its old functionality now lives under `Mesh.use_mirror_vertex_group_x`. Something went wrong in the UI in This transition. Differential Revision: https://developer.blender.org/D9287