cue: remove Merge

It has been deprecated and marked as DO NOT USE since April 2021,
which was even before the first tagged release, v0.0.2 in July 2021.
Now that it has been three full years, and the last use in 'cue cmd'
has been ripped out, it's time to remove it.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I4c03a3f9c767d29e62f9496c72ac06b74b7070e7
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1193719
Reviewed-by: Paul Jolly <paul@myitcv.io>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>

cue/instance.go

@@ -217,29 +217,6 @@ func (inst *hiddenInstance) Eval(expr ast.Expr) Value {
 	return v.Context().BuildExpr(expr, Scope(v), InferBuiltins(true))
 }
 
-// DO NOT USE.
-//
-// Deprecated: do not use.
-func Merge(inst ...*Instance) *Instance {
-	v := &adt.Vertex{}
-
-	i := inst[0]
-	ctx := newContext(i.index)
-
-	// TODO: interesting test: use actual unification and then on K8s corpus.
-
-	for _, i := range inst {
-		w := i.Value()
-		v.AddConjunct(adt.MakeRootConjunct(nil, w.v.ToDataAll(ctx)))
-	}
-	v.Finalize(ctx)
-
-	p := addInst(i.index, &Instance{
-		root: v,
-	})
-	return p
-}
-
 // Build creates a new instance from the build instances, allowing unbound
 // identifier to bind to the top-level field in inst. The top-level fields in
 // inst take precedence over predeclared identifier and builtin functions.