[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.

Author: philnik777

libcxx/include/__hash_table

@@ -808,7 +808,7 @@ public:
           }
           {
             __node_holder __h = __construct_node_hash(__hash, std::forward<_Args>(__args2)...);
-            if (size() + 1 > __bc * max_load_factor() || __bc == 0) {
+            if (size() + 1 > __bc * max_load_factor()) {
               __rehash_unique(std::max<size_type>(2 * __bc + !std::__is_hash_power2(__bc),
                                                   size_type(__math::ceil(float(size() + 1) / max_load_factor()))));
               __bc    = bucket_count();