fix: remove broken individual GraphFunction schema files

These individual .surql files in schema/functions/ were outdated, broken,
and conflicting with the correct implementations in schema/codegraph.surql:

- calculate_coupling_metrics.surql: Syntax error with orphaned code
- detect_circular_dependencies.surql: Wrong function name (fn::coupling_metrics)
- get_hub_nodes.surql: Wrong function signature
- get_reverse_dependencies.surql: Incomplete implementation
- get_transitive_dependencies.surql: Incomplete implementation
- trace_call_chain.surql: Incomplete implementation

The correct implementations in codegraph.surql use fn::node_info() helper
to return enriched node data (names, file paths, locations) instead of
bare record IDs.

Only codegraph.surql should be deployed via apply-schema.sh.

Author: jruokola