Remove old walk function

This removes the old global walk function, and changes all
the code to use the per-driver walk functions.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>

registry/storage/purgeuploads_test.go

@@ -142,7 +142,7 @@ func TestPurgeMissingStartedAt(t *testing.T) {
 	oneHourAgo := time.Now().Add(-1 * time.Hour)
 	fs, ctx := testUploadFS(t, 1, "test-repo", oneHourAgo)
 
-	err := Walk(ctx, fs, "/", func(fileInfo driver.FileInfo) error {
+	err := fs.Walk(ctx, "/", func(fileInfo driver.FileInfo) error {
 		filePath := fileInfo.Path()
 		_, file := path.Split(filePath)