bl_info = {
    "name": "Remove empty Vertex Groups",
    "category": "Mesh",
    "author": "CoDEmanX",
    "version": (1, 0),
    "blender": (2, 70, 0),
    "location": "Properties Editor > Object data > Vertex Groups > Specials menu",
    "description": "Delete Vertex Groups with no assigned weight of active object",
    "warning": "",
    "wiki_url": ""
}

import bpy
import bmesh

class RemoveEmptyVertexGroups(bpy.types.Operator):
    """Remove all empty vertex groups"""
    bl_idname = "mesh.remove_empty_vertex_groups"
    bl_label = "Remove empty Vertex Groups"
    bl_options = {'REGISTER', 'UNDO'}

    @classmethod
    def poll(cls, context):
        return (
            (context.object is not None) and
            (context.object.type == 'MESH')
        )

    def execute(self, context):
        ob = context.object
        ob.update_from_editmode()
        
        vgroup_used = {i: False for i, k in enumerate(ob.vertex_groups)}
        
        for v in ob.data.vertices:
            for g in v.groups:
                if g.weight > 0.0:
                    vgroup_used[g.group] = True
        
        for i, used in sorted(vgroup_used.items(), reverse=True):
            if not used:
                ob.vertex_groups.remove(ob.vertex_groups[i])
        
        return {'FINISHED'}

def draw_func(self, context):
    self.layout.operator(
        OBJECT_OT_vertex_group_remove_unused.bl_idname,
        icon='X'
    )

def register():
    #bpy.utils.register_module(__name__)
    #bpy.types.MESH_MT_vertex_group_specials.prepend(draw_func)
    bpy.utils.register_class(RemoveEmptyVertexGroups)

def unregister():
    #bpy.utils.unregister_module(__name__)
    #bpy.types.MESH_MT_vertex_group_specials.remove(draw_func)
    bpy.utils.unregister_class(RemoveEmptyVertexGroups)

if __name__ == "__main__":
    register()