Fix the Assign Automatic Weights operator in weight paint mode when the Vertex Groups X option is enabled. This issue was probably introduced in rB5502517c3c12 where `me->editflag & ME_EDIT_MIRROR_X` was replaced by either `me->editflag & ME_EDIT_VERTEX_GROUPS_X_SYMMETRY` or `me->symmetry & ME_SYMMETRY_X`. In this case, the former wasn't working, so I replaced it with the latter.