Merge duplicated --tcp option help text for lxd command

I found `lxd -h` prints `--tcp` option help text twice like below:

```
Usage: lxd [options]

Options:

    --tcp <addr:port>
        Bind to addr:port.
    --debug  (= false)
        Enables debug mode.
    --tcp (= "")
        TCP address to listen on in addition to the unix socket
    -v  (= false)
        Enables verbose mode.
```

With this pull request, it becomes like:

```
Usage: lxd [options]

Options:
    --debug  (= false)
        Enables debug mode.
    --tcp (= "")
        TCP address <addr:port> to listen on in addition to the unix socket (e.g., 127.0.0.1:443)
    -v  (= false)
        Enables verbose mode.
```