`:unlet $FOOBAR`

When an environment variable isn't set, `exists()` return false. We can assign a value to the variable with `:let`, after which point it exists. But we can't revert to an non-existent state (related to `unsetenv (3C)`)

``` vim
call assert_false(exists('$FOOBAR'))
let $FOOBAR = 1
call assert_true(exists('$FOOBAR'))
unlet $FOOBAR
call assert_false(exists('$FOOBAR'))
```
