cmd/internal/gc, cmd/internal/obj: remove pointless fmt.Sprintf calls

cmd/internal/gc, cmd/internal/obj: remove pointless fmt.Sprintf calls

This is a follow-up to CL 6265. No behavior changes.
The diff was generated with eg, using template:

package p

import "fmt"

func before(a string) string { return fmt.Sprintf(a) }
func after(a string) string  { return a }

Change-Id: I7b3bebf31be5cd1ae2233da06cb4502a3d73f092
Reviewed-on: https://go-review.googlesource.com/6269
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>