[libc++] Remove a redundant check from __hash_table::__emplace_unique (#162856)
The `|| __bc == 0` case will never be relevant, since we know that
`size() + 1` will always be exactly 1 if `__bc == 0` and `0 *
max_load_factor()` will be zero, so the branch will already be taken due
to the first condition.