The std::advance function has a clear precondition that it can only be
called with a negative distance when a bidirectional iterator is used.
However, prev() and next() don't have such preconditions explicitly,
they inherit it from calling advance().
This patch removes assertions in prev() and next() that were duplicates
of similar ones in advance(), and removes a copy-pasted comment that was
trying to justify the use of _LIBCPP_ASSERT_PEDANTIC but IMO is creating
confusion with little benefit.
d05ab119e1