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
90a23dec46