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

test/util.go

@@ -208,7 +208,7 @@ func alterFiles(dir string) error {
 							return err
 						}
 					} else {
-						err := osutil.Remove(path)
+						err := os.Remove(path)
 						if err != nil {
 							return err
 						}