'Armature' mode for the Mask Modifier was not working at all anymore even when
the selection <-> depsgraph recalc issue was patched to work (this latter fix is
coming in another commit). It appears that this probably happened during one or
more of the refactors which may have taken place around here over the years
since I first introduced it.
This commit does two things:
* Removed the unused/redundant "vgroupHash"
* Fixed the incorrect assumption used for determining if the vertex actually
belonged to a vgroup corresponding to a selected bone.
1d4213b2bc