SI-5: Deprecate rest-api-core and remove from workspace

[josecelano]

Subissue of EPIC #1938 — REST API Contract-First Migration

Problem

After SI-1 through SI-4 migrate all contexts, rest-api-core (torrust-tracker-rest-api-core) becomes an empty shell. Its only consumer is axum-rest-api-server — once SI-4 removes the last stats dependency, the crate has no reason to exist.

Prerequisites

• SI-4 completed (last consumer of rest-api-core types removed)

Scope

• Remove torrust-tracker-rest-api-core from axum-rest-api-server/Cargo.toml
• Remove crate from workspace Cargo.toml members
• Delete packages/rest-api-core/ directory
• Update deny.toml if crate had wrapper rules
• Verify no build/test breakage

Checklist

• No crate references torrust-tracker-rest-api-core
• Workspace builds cleanly
• Integration tests pass
• Pre-commit and pre-push checks pass