fix own error with removing vertex weights not checking for non-existing weights.
however this exposed some other odd behavior.

removing a vertex group would add one if not found but only for meshes, not lattices.

now just skip removal if not found.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57239 954f8c5b-7b00-dc11-b283-0030488c597c