Revert "git-commit-auto-mode-enable: don't call on load and autoload"

This reverts commit eff561236e01102232a1c2bb4a1840175ead1368.
Revert "git-commit-auto-mode-{enable,disable,regexps}: new fns, var"

This reverts commit c0f85b63cc4e63ea3a787c109ce6d32935ffa2fb.
git-commit-auto-mode-alist: define as noop

This avoids problems when users don't update magit and git-commit-mode
at the same time.