refactor(tvix/eval): Drop LightSpan entirely

This was made unnecessary in c92d06271 (feat(tvix/eval): drop
LightSpan::Delayed, 2023-12-08) because it didn't improve benchmarks as
much as expected and has been vestigial since; this continues the
cleanup by just removing it altogether

Change-Id: I21ec7ae9b52a5cccd2092696a5a87f658194d672
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11949
Autosubmit: aspen <root@gws.fyi>
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/eval/src/vm/macros.rs

@@ -49,7 +49,7 @@ macro_rules! cmp_op {
                     }
                 }
 
-                let gen_span = $frame.current_light_span();
+                let gen_span = $frame.current_span();
                 $vm.push_call_frame($span, $frame);
                 $vm.enqueue_generator("compare", gen_span, |co| compare(a, b, co));
                 return Ok(false);