SOC-4346 | [REST API] Space activities service should destroy the specif...

...ied identity like and not activity owner like

**Problem Analysis**:
* ```org.exoplatform.social.service.rest.ActivitiesRestService#destroyLike(java.lang.String, java.lang.String)``` method actually loads the activity owner as an identity for whom to delete the like:
```
getIdentityManager().getIdentity(activity.getUserId(), false);
```

**Fix Description:**
* Use the ```identityId``` method argument to load the right identity.