Remove lagecy vtcode-core/src/llm/llm_modular/providers/ module

[vinhnx]

Description

Remove lagecy vtcode-core/src/llm/llm_modular/providers/ module

Environment

• OS: [e.g. macOS 14.1, Ubuntu 22.04, Windows 11]
• VT Code version: [e.g. 0.31.0 or commit hash]
• Rust version: [e.g. rustc 1.75.0]

Additional Context

Add any other context about the issue or question here.