docs: remove vectorless-code documentation

Removed the entire vectorless-code documentation file that described
the tree-traversal based code search implementation. This included
details about the three-tier query strategy (Fast/Standard/Deep modes),
comparison with existing tools like cocoindex-code and codeindex,
architecture diagrams, and the planned work breakdown for implementing
code format support in vectorless with keyword search API and
layered traversal queries.

Author: zTgx