lib: Remove osutil.Remove & osutil.RemoveAll (fixes #3513)

These are no longer required with Go 1.7. Change made by removing the
functions, doing a global s/osutil.Remove/os.Remove/.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3514

lib/versioner/staggered.go

@@ -165,7 +165,7 @@ func (v Staggered) expire(versions []string) {
 			continue
 		}
 
-		if err := osutil.Remove(file); err != nil {
+		if err := os.Remove(file); err != nil {
 			l.Warnf("Versioner: can't remove %q: %v", file, err)
 		}
 	}