Remove unnecessary _LIBCPP_STRING_INTERNAL_MEMORY_ACCESS (#79574)

This macro is unnecessary with `basic_string& operator=(value_type
__c)`.

Author: AdvenamTacet

libcxx/include/string

@@ -1138,7 +1138,7 @@ public:
 #if _LIBCPP_STD_VER >= 23
   basic_string& operator=(nullptr_t) = delete;
 #endif
-  _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_STRING_INTERNAL_MEMORY_ACCESS basic_string& operator=(value_type __c);
+  _LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string& operator=(value_type __c);
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 iterator begin() _NOEXCEPT {
     return __make_iterator(__get_pointer());