Remove XEmacs compat code from allout.el

Remove XEmacs compat code from allout.el

* lisp/allout.el (allout-overlay-preparations)
(allout-overlay-interior-modification-handler)
(allout-before-change-handler, allout-beginning-of-line)
(allout-solicit-alternate-bullet, allout-annotate-hidden)
(allout-hide-by-annotation, allout-yank-processing)
(allout-flag-region, allout-toggle-subtree-encryption)
(allout-mark-marker, allout-substring-no-properties)
(allout-select-safe-coding-system)
(allout-previous-single-char-property-change)
(allout-next-single-char-property-change)
(top-level): Remove XEmacs compat code.