Revert "imports: wait for fastWalk workers to finish before returning"

This reverts commit 4436e54754.

Reason for revert: Breaks goimports. See:
https://github.com/golang/go/issues/16399#issuecomment-293248363

Change-Id: I3bda8f0fd32380d19d7daecf3489a24e51abfbe7
Reviewed-on: https://go-review.googlesource.com/40296
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

imports/fastwalk.go

@@ -19,7 +19,6 @@ import (
 	"os"
 	"path/filepath"
 	"runtime"
-	"sync"
 )
 
 // traverseLink is a sentinel error for fastWalk, similar to filepath.SkipDir.
@@ -49,12 +48,6 @@ func fastWalk(root string, walkFn func(path string, typ os.FileMode) error) erro
 	if n := runtime.NumCPU(); n > numWorkers {
 		numWorkers = n
 	}
-
-	// Make sure to wait for all workers to finish, otherwise walkFn could
-	// still be called after returning.
-	var wg sync.WaitGroup
-	defer wg.Wait()
-
 	w := &walker{
 		fn:       walkFn,
 		enqueuec: make(chan walkItem, numWorkers), // buffered for performance
@@ -67,8 +60,7 @@ func fastWalk(root string, walkFn func(path string, typ os.FileMode) error) erro
 	defer close(w.donec)
 	// TODO(bradfitz): start the workers as needed? maybe not worth it.
 	for i := 0; i < numWorkers; i++ {
-		wg.Add(1)
-		go w.doWork(&wg)
+		go w.doWork()
 	}
 	todo := []walkItem{{dir: root}}
 	out := 0
@@ -111,8 +103,7 @@ func fastWalk(root string, walkFn func(path string, typ os.FileMode) error) erro
 
 // doWork reads directories as instructed (via workc) and runs the
 // user's callback function.
-func (w *walker) doWork(wg *sync.WaitGroup) {
-	defer wg.Done()
+func (w *walker) doWork() {
 	for {
 		select {
 		case <-w.donec: