ci(v2): remove the benchmark workflow

The v2 workflow predates the branch gates and published its results into
the same gh-pages benchmarks/data.js as main on every push, mixing the
internal v2 utils benchmarks into the v3 charts. Its PR comparisons ran
against that mixed baseline, so they were misleading as well. v3 benchmarks
are covered by the centralized workflow in gofiber/.github.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: ReneWerner87