Remove placeholder runtime MCP manager

[fredporter]

Summary

• remove the empty persisted MCP server registry from uCode UDO runtime
• remove fake start, stop, status, and tool-call methods
• remove the four corresponding Flask routes
• stop advertising the false ThinUI endpoint and future Rust implementation

External Model Context Protocol remains owned by the uCore udos-mcp gateway.

Verification

• scoped audit finds no removed manager symbols or routes
• full Python matrix: 198 passed, 33 existing skips, 2 existing warnings