internal/core/adt: remove unused nodeContext fields

nodeContext's overlays and disjunct fields were added and set in
https://cuelang.org/cl/1190800 but were never used.

Peak memory usage on a simplified version of
https://github.com/uhthomas/automata goes down a bit,
as most of the allocations come from cloning vertices and contexts,
so the size of these types is multiplied:

                    │      old       │                 new                  │
                    │ peak-RSS-bytes │ peak-RSS-bytes  vs base              │
    VetAutomataList     669.5Mi ± 3%     629.0Mi ± 3%  -6.06% (p=0.000 n=8)

Updates #3334.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I7ebf8db1f72cee7e4246ad10c4c0d9864ea3110e
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1213518
Reviewed-by: Marcel van Lohuizen <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: mvdan

internal/core/adt/disjunct2.go

@@ -476,8 +476,6 @@ func (n *nodeContext) doDisjunct(c Conjunct, m defaultMode, mode runMode, orig *
 	// a special mode, or evaluating more aggressively if finalize is not given.
 	v.status = unprocessed
 
-	d.overlays = n
-	d.disjunct = c
 	d.scheduleConjunct(c, c.CloseInfo)
 
 	oc.unlinkOverlay()

internal/core/adt/eval.go

@@ -1027,10 +1027,6 @@ type nodeContext struct {
 	// set for all Vertex values that were cloned.
 	underlying *Vertex
 
-	// overlays is set if this node is the root of a disjunct created in
-	// doDisjunct. It points to the direct parent nodeContext.
-	overlays *nodeContext
-
 	nodeContextState
 
 	scheduler
@@ -1098,7 +1094,6 @@ type nodeContext struct {
 	disjuncts    []*nodeContext
 	buffer       []*nodeContext
 	disjunctErrs []*Bottom
-	disjunct     Conjunct
 
 	// hasDisjunction marks wither any disjunct was added. It is listed here
 	// instead of in nodeContextState as it should be cleared when a disjunction