Implement API endpoint for deleting a merge request dependency

In order for an API consumer to manage merge request dependencies, API endpoints should be implemented. Currently, they are only manageable via the web UI.

Proposal

This issue is specific for implementing endpoint for deleting a merge request dependency: DELETE /projects/:id/merge_requests/:iid/blocks/:block_id.

Question to be confirmed:

• Should we allow deletion of dependency when user can't read the blocking MR but can update the blocked MR?