all: remove redundant interface type assertions

These two are of the form:

	type T interface { ... }
	var v1 T
	v2 := v1.(T)

That is, if v1 is already of interface type T,
then the type assertion `.(T)` is unnecessary.
Found via staticcheck's S1040.

Change-Id: Icd57e251121cb1f646106115ab5c9f9cb40768bb
Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/536097
Unity-Result: CUEcueckoo <cueckoo@cuelang.org>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>

internal/core/adt/eval.go

@@ -1163,7 +1163,7 @@ func (n *nodeContext) getValidators() BaseValue {
 		v = &BasicType{K: n.kind}
 
 	case 1:
-		v = a[0].(Value) // remove cast
+		v = a[0]
 
 	default:
 		v = &Conjunction{Values: a}

pkg/internal/errors.go

@@ -77,21 +77,15 @@ func (c *CallCtxt) invalidArgType(arg adt.Value, i int, typ string, err error) {
 		c.Err = b
 		return
 	}
-	v, ok := arg.(adt.Value)
 	// TODO: make these permanent errors if the value did not originate from
 	// a reference.
-	if !ok {
-		c.errf(nil,
-			"cannot use incomplete value %s as %s in argument %d to %s",
-			arg, typ, i, c.Name())
-	}
 	if err != nil {
 		c.errf(err,
 			"cannot use %s (type %s) as %s in argument %d to %s",
-			arg, v.Kind(), typ, i, c.Name())
+			arg, arg.Kind(), typ, i, c.Name())
 	} else {
 		c.errf(err,
 			"cannot use %s (type %s) as %s in argument %d to %s",
-			arg, v.Kind(), typ, i, c.Name())
+			arg, arg.Kind(), typ, i, c.Name())
 	}
 }