internal/core/adt: revert fix for 2209

This seemed to cause more harm than good.
A fix for 2209 is planned for an upcoming evaluator
restructuring.

Fixes #2263
Fixes #2246

Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Change-Id: Ia6978abc59f1c1cbd371fd636e719859593f19e6
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/551136
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Unity-Result: CUEcueckoo <cueckoo@cuelang.org>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
(cherry picked from commit b300e4f)
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/551151

cue/testdata/benchmarks/issue1684.txtar

@@ -34,14 +34,14 @@ out: #secrets & {
 }
 -- out/eval/stats --
 Leaks:  0
-Freed:  4599
-Reused: 4579
-Allocs: 20
+Freed:  995025
+Reused: 994976
+Allocs: 49
 Retain: 0
 
-Unifications: 3207
-Conjuncts:    16425
-Disjuncts:    4599
+Unifications: 740771
+Conjuncts:    3143640
+Disjuncts:    995025
 -- out/eval --
 (struct){
   #Secret: (#struct){