[Feature] Deprecate Rosetta MCP - we only use plugin

[awrobel-gd]

Problem

We have code paths functionally dead that complicate many places in code.

Proposed Solution

Remove MCP modes, all mentions of Rosetta configs, all comments explaining Rosetta MCP vs plugin.

Component

Where would this live? (e.g. backend, mcp_server, TUI/CLI, docs)
backend

Additional Context

This is cleanup after fully migrating to plugin of Rosetta -> see Dockerfile in backend.