ci: remove auto release workflow

[BeiBaiNian]

Summary

Related: apache/casbin#1747

As requested in apache/casbin#1747,
repositories should stop auto-publishing new versions to GitHub Releases and
package registries. New releases will instead be voted on by the community and
published manually via
incubator-release-assistant.

Changes in this PR:

• remove .github/workflows/release.yml, which triggered on every v* tag to
  run cargo publish (crates.io) and create a GitHub Release automatically
• keep the CI pipeline (ci.yml: clippy, fmt, build and the
  PostgreSQL / MySQL / SQLite test matrix) and the coverage workflow unchanged

No Rust source code or Cargo.toml is touched.

Verification

• cargo test --no-default-features --features sqlite,runtime-async-std-native-tls
  passes locally: 3 passed, 0 failed (after setting DATABASE_URL=sqlite:casbin.db
  and creating the casbin_rule table, same as CI)
• clippy / fmt / build and the full database test matrix are to be confirmed by
  GitHub Actions on this PR