Delete the BcStackValues trait

Summary: Not used.

Reviewed By: JakobDegen

Differential Revision: D54553071

fbshipit-source-id: 35046b5331774bf319b6e261713b9b348be50eaf

Author: ndmitchell

starlark/src/eval/bc/mod.rs

@@ -34,5 +34,4 @@ pub(crate) mod opcode;
 pub(crate) mod repr;
 pub(crate) mod slow_arg;
 pub(crate) mod stack_ptr;
-pub(crate) mod stack_values;
 pub(crate) mod writer;