Remove XEmacs compat code from speedbar.el

Remove XEmacs compat code from speedbar.el

* lisp/speedbar.el (speedbar-frame-plist, speedbar-frame-mode)
(speedbar-make-overlay, speedbar-overlay-put, speedbar-delete-overlay)
(speedbar-mode-line-update, speedbar-frame-reposition-smartly)
(speedbar-set-mode-line-format, speedbar-reconfigure-keymaps)
(speedbar-add-localized-speedbar-support, speedbar-check-vc)
(speedbar-highlight-one-tag-line)
(speedbar-unhighlight-one-tag-line): Remove XEmacs compat code.

* doc/misc/speedbar.texi (Frames and Faces): Remove documentation for
XEmacs.