Bind <Leader>wC to `delete-other-windows

As per the Vim documentation for `CTRL-W_o`:

```
|CTRL-W_o|      CTRL-W o           close all but current window (like |:only|)
```

Since the `<Leader>` bindings for `w` closely mirror `CTRL-W` bindings, it would be nice to maintain `wo` as `delete-other-windows`. To keep `other-frame`, I rebound it to `wO`.