Remove default parameters from z_closure

[sashacmc]

The default parameters for z_closure not implemented in zenoh-c. It's possible to add it (done in PR https://github.com/eclipse-zenoh/zenoh-c/pull/782) , but in fact this functionality is useless in practice: no one will use callbacks without context in real applications. So for simplicity it seems better to remove this, despite that this is slight API change.

[github-actions[bot]]

PR missing one of the required labels: {'dependencies', 'breaking-change', 'bug', 'internal', 'documentation', 'new feature', 'enhancement'}