`ungroup()` doesn't respect dots

[mgirlich]

ungroup.dtplyr_step() always removes all groups instead of only the specified ones

library(dtplyr)
library(dplyr, warn.conflicts = FALSE)

dt <- lazy_dt(data.frame(x = 1, y = 1))
dt %>% 
  group_by(x, y) %>% 
  ungroup(x) %>% 
  group_vars()
#> character(0)

dt %>% 
  collect() %>% 
  group_by(x, y) %>% 
  ungroup(x) %>% 
  group_vars()
#> [1] "y"

^{Created on 2021-05-25 by the reprex package (v2.0.0)}