go/build: remove use of package log

package log does not belong here,
even for an impossible condition.

Change-Id: I6180a4f7766a8e5759db4f33d03703d0b678d0b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/241077
Run-TryBot: Russ Cox <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: rsc

src/go/build/build.go

@@ -16,7 +16,6 @@ import (
 	"internal/goversion"
 	"io"
 	"io/ioutil"
-	"log"
 	"os"
 	"os/exec"
 	pathpkg "path"
@@ -924,7 +923,7 @@ Found:
 				quoted := spec.Path.Value
 				path, err := strconv.Unquote(quoted)
 				if err != nil {
-					log.Panicf("%s: parser returned invalid quoted string: <%s>", filename, quoted)
+					panic(fmt.Sprintf("%s: parser returned invalid quoted string: <%s>", filename, quoted))
 				}
 				fileImports = append(fileImports, importPos{path, spec.Pos()})
 				if path == "C" {