Hard-errors if any column still carries the haven_labelled class, i.e.
a raw labelled column read from a .dta/.sav that was never
recoded or stripped. A composable primitive used by check_schema and
reusable by bespoke checkers.
See also
Other schema assertions:
assert_key_unique(),
assert_schema(),
check_schema()
Examples
assert_no_labelled(data.frame(x = 1:3))