math/big: delete solved TODO

The TODO is no longer needed as it was solved by a previous CL.
See https://go-review.googlesource.com/14995.

Change-Id: If62d1b296f35758ad3d18d28c8fbb95e797f4464
Reviewed-on: https://go-review.googlesource.com/65232Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/big/intconv.go

@@ -12,12 +12,6 @@ import (
 	"io"
 )
 
-// TODO(gri) Should rename itoa to utoa (there's no sign). That
-// would permit the introduction of itoa which is like utoa but
-// reserves a byte for a possible sign that's passed in. That
-// would permit Int.Text to be implemented w/o the need for
-// string copy if the number is negative.
-
 // Text returns the string representation of x in the given base.
 // Base must be between 2 and 36, inclusive. The result uses the
 // lower-case letters 'a' to 'z' for digit values >= 10. No base