go/types, constant: remove superfluous import comment

The comment was a left-over from the long-past move
of these two packages from x/tools to the std lib.

Fixes #21791.

Change-Id: I65cbebf479e609be0204b58edb6506c6403aec9b
Reviewed-on: https://go-review.googlesource.com/64250
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/constant/value.go

@@ -10,7 +10,7 @@
 // values produce unknown values unless specified
 // otherwise.
 //
-package constant // import "go/constant"
+package constant
 
 import (
 	"fmt"

src/go/types/api.go

@@ -24,7 +24,7 @@
 //
 // For a tutorial, see https://golang.org/s/types-tutorial.
 //
-package types // import "go/types"
+package types
 
 import (
 	"bytes"