`ungroup(lf, x)` should only remove `x` from group

[mgirlich]

Instead of only removing x from the groups it removes all grouping

library(dplyr, warn.conflicts = FALSE)
library(dbplyr)
#> 
#> Attaching package: 'dbplyr'
#> The following objects are masked from 'package:dplyr':
#> 
#>     ident, sql

lazy_frame(x = 1, y = 2) %>% 
  group_by(x, y) %>% 
  ungroup(x) %>% 
  group_vars()
#> character(0)

^{Created on 2021-08-16 by the reprex package (v2.0.1)}