The code in Close() that removes the watches was not working,
because it first sets `w.closed = true` and then calls w.close(),
which starts with
```
if w.closed {
return errPollerClosed
}
```

Fix by setting w.closed only after calling w.remove() for all the
files being watched.

While at it, remove the duplicated `delete(w.watches, name)` code.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>