As part of the project of converting `MVert` into `float3` (more details in T93602), this is an easy step, since it is only locally used runtime data. In the six places it was used, the flag was replaced by a local bitmap. By itself this change has no benefits other than making some code slightly simpler. It only really matters when the other flags are removed and it can be removed from `MVert` along with the bevel weight. Differential Revision: https://developer.blender.org/D13878