Don't warn generally for `IO::Buffer`, only on specific code paths e.g. `map`. (#9131)

io_buffer.c

@@ -699,8 +699,6 @@ io_flags_for_size(size_t size)
 VALUE
 rb_io_buffer_initialize(int argc, VALUE *argv, VALUE self)
 {
-    io_buffer_experimental();
-
     rb_check_arity(argc, 0, 2);
 
     struct rb_io_buffer *buffer = NULL;