[ADT] Drop append_range test that doesn't compile with EXPENSIVE_CHECKS

The issue seems to be caused by the definition of `SafeIntIterator` and
is otherwise unrelated to the implementation of `append_range`.

We have a simialr test below, so this does not meaningfully decrease the
test coverage of `append_range`.

See the discussion in: https://reviews.llvm.org/D144420#4164373

Issue: https://github.com/llvm/llvm-project/issues/61122