staticd: Add `no` form for `static-sids` command

staticd: Add `no` form for `static-sids` command

Currently, when the user tries to delete all static SIDs with the
`no static-sids` command, staticd returns an error.

```
router# config
router(config)# segment-routing
router(sr)# srv6
router(srv6)# no static-sids
% Unknown command: no  static-sids
```

The problem is the `static-sids` command does not support the `no` form.

This PR enables the `no` form for the `static-sids` command.

```
router# config
router(config)# segment-routing
router(sr)# srv6
router(srv6)# no static-sids
```

Signed-off-by: Carmine Scarpitta <cscarpit@cisco.com>