[libc++] Remove unused variable

[fanbo-meng]

Summary

Removing __current as it becomes unused-but-set after 2cf78d4ead4a2ab5375bd6087724211d04119a28.

libcxx/include/regex

if (__st)
         __states.back().__first_ = __first;
         __states.back().__current_ = __first;
         __states.back().__last_ = __last;
         __states.back().__sub_matches_.resize(mark_count(), __unmatched);
         __states.back().__loop_data_.resize(__loop_count());
         __states.back().__node_ = __st;
         __states.back().__flags_ = __flags;
         __states.back().__at_first_ = __at_first;
-        const _CharT* __current = __first;
         bool __matched = false;
         int __counter = 0;
         int __length = __last - __first;
         do
         {
             ++__counter;
             if (__counter % _LIBCPP_REGEX_COMPLEXITY_FACTOR == 0 &&
                 __counter / _LIBCPP_REGEX_COMPLEXITY_FACTOR >= __length)
if (__st)
                 }
                 __matched = true;
                 if (__highest_j == _Np)
                     __states.clear();
                 else
                     __states.pop_back();
                 break;
             case __state::__accept_and_consume:
-                __current = __s.__current_;
-                break;
             case __state::__repeat:
             case __state::__accept_but_not_consume:
                 break;
             case __state::__split:
                 {
                 __state __snext = __s;
                 __s.__node_->__exec_split(true, __s);
                 __snext.__node_->__exec_split(false, __snext);