Add `invalid_remove()`

Just `x[!invalid_detect(x)]`, but it would be nice in a pipe based workflow. Could have used it here https://twitter.com/dvaughan32/status/1388463791596265473?s=20

```r
year_month_weekday(grid$year, grid$month, grid$day, grid$index) %>%
  invalid_remove() %>%
  as.Date() %>%
  sort()
```