Remove cut buffer from documentation.

Remove cut buffer from documentation.

* doc/emacs/frames.texi (Cut/Paste Other App): Remove vut-buffer text.

* doc/lispref/text.texi (Low-Level Kill Ring):
* doc/lispref/frames.texi (Window System Selections): Remove cut buffer
documentation.

* doc/misc/cl.texi (Basic Setf): Remove x-get-cut-buffer and x-get-cutbuffer.

* lisp/term/x-win.el (x-select-text):
* lisp/term/pc-win.el (x-selection-value):
* lisp/term/ns-win.el (x-selection-value):
* lisp/eshell/em-term.el:
* lisp/w32-fns.el (x-get-selection-value):
* lisp/mouse-sel.el (mouse-sel-set-selection-function):
* lisp/frame.el (display-selections-p): Remove cut-buffer in documentation.

* lisp/term.el (term-mouse-paste): Don't call x-get-cutbuffer.
Remove cut buffer from error message.