po-mode: Remove support for Emacs versions < 22.
Emacs 22 is the oldest version still in use, see
<https://repology.org/project/emacs/versions>.
* gettext-tools/emacs/po-compat.el: Remove conditional code for Emacs < 22.
* gettext-tools/emacs/po-mode.el: Likewise.
(po-rehighlight): Remove function.
(po-check-all-pending-edits, po-check-for-pending-edit): Update.
* gettext-tools/emacs/start-po.el: Update comment.
