chore(tvix/eval): remove existing nested key implementation

This implementation, which only ever worked for non-recursive
attribute sets, is no longer needed and thus removed here.

We have a new implementation of these nested keys coming up instead.

Change-Id: I0c2875154026a4f5f6e0aa038e465f54444bf721
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6783
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/opcode.rs

@@ -91,7 +91,6 @@ pub enum OpCode {
     /// Note that this takes the count of *pairs*, not the number of *stack values* - the actual
     /// number of values popped off the stack will be twice the argument to this op
     OpAttrs(Count),
-    OpAttrPath(Count),
     OpAttrsUpdate,
     OpAttrsSelect,
     OpAttrsTrySelect,