Generic method for removing elements of object

`rmvar(x, ...) <- value`

- x
Model object

- ...
additional arguments to lower level functions

- value
Vector of variables or formula specifying which nodes to remove

`cancel`

```
m <- lvm()
addvar(m) <- ~y1+y2+x
covariance(m) <- y1~y2
regression(m) <- c(y1,y2) ~ x
### Cancel the covariance between the residuals of y1 and y2
cancel(m) <- y1~y2
### Remove y2 from the model
rmvar(m) <- ~y2
```