docs: remove mention of Rizin cmd calls

[XVilka]

Your checklist for this pull request

• I've read the guidelines for contributing to this repository
• I made sure to follow the project's coding style
• I've updated the documentation with the relevant information (if needed)

Detailed description

Now, that we migrated almost all Rizin command calls to use the API, time to remove the mention of it from the documentation. The code itself not removed (yet), but it is discouraged to add new calls anyway.

Test plan (required)

Change makes sense.