internal/imports: don't set a logger unless the user has provided it

Not sure why I thought it was a good idea to enable extra logging
by default, but this was added in CL 184198.

Fixes golang/go#37636

Change-Id: I1840a9e53625db99c9097f2c23500ae20d6dac1f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/221918
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

internal/imports/imports.go

@@ -21,7 +21,6 @@ import (
 	"go/token"
 	"io"
 	"io/ioutil"
-	"log"
 	"os"
 	"regexp"
 	"strconv"
@@ -155,12 +154,6 @@ func initialize(filename string, src []byte, opt *Options) ([]byte, *Options, er
 			GOSUMDB:     os.Getenv("GOSUMDB"),
 		}
 	}
-
-	// Set the logger if the user has not provided it.
-	if opt.Env.Logf == nil {
-		opt.Env.Logf = log.Printf
-	}
-
 	if src == nil {
 		b, err := ioutil.ReadFile(filename)
 		if err != nil {