🔥 Exterminate legacy backport GHA workflow

Previously, this workflow was being used to cherry-pick PRs made
against `main` into older stable branches. Now that pytest-dev#12475 integrated
the Patchback GitHub App, it's no longer needed as it's making
duplicate pull requests which don't even trigger CI runs automatically
due to pytest-dev#10354. So this patch removes said workflow to address the
problem.